This means worlds with varying levels of technology and magic—cultures existing side by side, perhaps, but still not on the same technical level.
1) Geographical barriers. If you really want a juxtaposed world, with no chance that one culture could acquire tools or know-how magical or mechanical from another, the obvious choice is to put the two on either side of an immense geographical barrier. The Spanish Empire and the Aztec Empire were not on the same technological footing when they met, both because of the ocean between them and because of evolutionary history. (See point 2). The sea voyage between them was rough and long, and before Spain came together as a nation enough to look outside its own borders, it had no reason to go sailing off to the other side of the world anyway. Saying “Two cultures can’t exist in the same world if one has spears and the other has airships!” is entirely silly, and ignores historical example.
Of course, perhaps you have a continent where two cultures live side by side. (This is the situation in almost all fantasy worlds. Most authors have this extremely odd bias against writing about a world with more than two continents, even when the world is post-medieval and possesses the sailing technology and commercial motives to cross oceans). Then it might seem as though their not sharing technology is impossible.
Geographical barriers are still a problem. How fast do airships go? I would bet you not as fast as jet planes. A journey to the other side of an inland sea or a mountain range would still be a problem, especially since those barriers would influence the wind in their own ways, and could be a graveyard for crashes. And once the airships landed in the other culture’s territory, either there would have to be free trade of secrets—and what’s the motive for that?—or the exact right person would have to see the airship and figure out its workings on his or her own. It might not be plausible to extend this situation for hundreds of years without human influence (see point 3), but it certainly isn’t as though two cultures are going to meet and one is going to acquire all the other’s different tools right that second, by osmosis.
There are also underground and undersea cultures. Most authors portray these, when they show up, as horrifically backwards, or technologically advanced but fallen. Why? It could just as easily be the other way around. Perhaps the undersea culture, given the right impetus, has submarines and periscopes and procedures to pull up useful minerals from the ocean bottom when the land cultures are just beginning to think that wheels might be a good idea.
2) Nature doesn’t just put the right raw materials wherever somebody wants them. Evolutionary history dictated that there were no horses in North America for a long time before the Spanish brought them back over; they had existed on the continent at one point, but became extinct. So the Aztecs didn’t have them, and they were a factor, though not as great as disease or gunpowder, in letting the Spanish conquer them.
I have a very hard time thinking of an analogous situation in a fantasy novel. Science fantasy, yes; Marion Zimmer Bradley’s Darkovans turn to psychic powers and technology based on the planet’s native crystals precisely because Darkover is poor in a lot of the metals, like iron, that led Earth to its own brand of science. (Darkover is also a harsher planet than Earth is in sheer extremes of weather, and colder thanks to its red sun, and those influence the lifestyle too). But a lot of fantasy novels seem to assume that not only does everyone ride horses, everyone has horses to ride. And of course everyone has access to iron in order to forge steel swords, and access to gold to make currency out of it, and access to wood to burn…
This is where fantasy would benefit from being dragged—kicking and screaming, if that’s really necessary—out of the pseudo-European background that’s the generic template right now. Start thinking about what that particular culture, given its setting, is going to use for weapons, for money, for transportation, for food. If you really aren’t sure, then go and study analogous situations on Earth until you do know. There’s no shortage of them out there. But generic fantasy has slammed a set of assumptions so deeply into many authors’ heads that they never bother questioning them.
…Yeah, and back to the point of the rant, which is that a juxtaposed world is indeed plausible if one country is poor or rich in some essential material. Get used to thinking in terms of limits, rather than ways to bypass those limits, and adaptation, rather than reshuffling the world via magic every time the character might have a problem. No, it doesn’t make sense for there to be huge variations in the levels of technology if everyone has equal access to raw materials and equal skill in using them. But all you have to do is stop permitting everybody free access, and the embarrassing unlikelihood of that scenario goes down.
However, the potential for conflict goes up like whoa.
3) “Can I have your secret of airships?” “No.” Different cultures may be committed to keeping their technological or magical secrets, well, secret, especially if they’re the only reason that that particular group has an economic, political, military, or theological edge. The more complicated the process, the more scattered it is, and the more coordination it involves among different people and operations, the less likely that someone lucky could just sneak in from another culture and “steal” it. Even when someone does have the technical knowledge, it can still be very tricky to turn theory into practice. And when the possessive culture actively discourages other people from prying, in the same way that guilds or schools of mages might try to keep their secrets…well. Someone can be killed for what he knows, even if he’s “innocently” trying to improve life for a bunch of other people.
This can range from the very simple (such as selling only the geldings of a certain kind of horse, so that the buyers have no possibility of breeding any of that kind of horse on their own) to the complicated (keeping the process of building a steam engine secret). Sure, other cultures can overcome those precautions, and in fact it would be entertaining to watch conflicts at that level rather than the “call out the armies right now, because we’re all marching off to die” level. But no culture can change overnight because of one added process they didn’t possess before, and no culture that benefits enormously from its own discoveries should just walk around handing them out like candy.
4) The “progress” ideal is not universal, thank you. We might assume it is, but that’s because it tends to be embedded so deeply into twenty-first-century Western culture. For example, think about rationing your electricity so as to conserve resources, or because other people need electricity, too, and you can only have two hours a day. The thought will probably make your skin crawl. Thus the push for “alternative energy” resources, and expanding the wealthy sector of society rather than limiting from the top down and building from the bottom up, and the sharp resistance to notions of cutting back, of limits, of ending. The “onward and upward” push is there. Progress, advancement, forward-looking… all of the terms imply not only continuation, but better continuation. Any change should be beneficial change, and beneficial change only. Many science fiction novels about the future of Earth, whether or not Earth is now part of a galactic empire, envision everyone as having equal access to things like the far-future cousin of the Internet, never mind “basic” necessities like the equivalent of television and washing machines. The most usual exception is a small elite who have access, not to a more technologically limited lifestyle, but to technology quantum leaps beyond our current dreams, and also to unimaginable leisure and wealth. And, most of the time, the people who don’t have access to any of those three want it, and should have it.
(Right here, I’d like to stage a small protest against people who use “evolution” in this sense. Evolution does not mean that, thank you. There’s no guarantee that life-forms now on Earth are any “better” than the ninety-nine percent who are dead, in terms of lifespan, killing capacity, or right to exist. They are the species who survived the conditions in the past, or the descendants of species who survived the conditions in the past. It doesn’t mean that new species may not arise, nor that every species now alive will always continue, forever and ever, amen. Thank you. /puts down bullhorn)
It might be very hard for science fiction to plausibly shed that attitude, since so many of them are supposed to have developed out of our own present-day Western culture. Fantasy cultures are different. I’d like to see someone try creating one without that deeply-embedded ideal of progress, and without portraying that culture as horribly “backward” (see! See what terms like that imply?) and “primitive” (another one) for being so.
There have been cultures like that in Earth history—including “Western” cultures in earlier stages. I’ve heard some arguments against this that go, “Oh, but it’s only human to strive after progress, and any culture that remains non-progressive next to a progressive one is stagnant!” Bullshit. We only think it’s natural because it’s everywhere in our own society, and it’s hard thinking another way.
Good. Make it hard. Create a culture you have to struggle and stretch to imagine the inside of. I’ll be perfectly willing to wait, since I think this is the most challenging, and thus fascinating, way to conceive a juxtaposed world.
5) Magical variation. Perhaps, on the eastern side of the Sunset Mountains, airships work just fine. But fly them over the Sunset Mountains to the west, and poof goes the technology that drives the airship, because the magic living to the west is sentient, and bored, and mischievous, and loves nothing so much as to get into the airship’s gears and muck things up. Ooh, pretty crashes! It’ll probably take the people who live on either side of the Sunset Mountains a while to solve that one, and meanwhile, you have a juxtaposed world.
This doesn’t have to be tied to geography, either. It can be tied to the wielders of magic. Jim Butcher’s wizards, in his Dresden Files series, have to live without most technology. Electricity goes whacko around them, computers get fried, refrigerators and televisions won’t work, security cameras blur, guns misfire, they need to use fire in lieu of central heating, and it’s not a sure bet of whether the hero’s car is going to get him from home to a house three blocks away. Other magical creatures suffer the same problem/benefit, so that a “conclusive” videotape of a werewolf looks all blurry because of the werewolf’s magical aura blurring it. To Butcher’s credit, he treats this not just as a free ride for his villains to get away with petty theft, but to disadvantage his good guys badly—and have them irritate their friends, as when Harry Dresden, the books’ main character, explodes his best friend’s hard drive by wandering into her office.
See? Once again, the trick is to keep your eye on the limitations, not just on all the cool things that might happen if some people stuck to technology and some to magic. Also, this option is last on the list for a very good reason. NO CHEATING. If you can come up with a way to create a juxtaposed world other than using magic, it’d be best. If you have to use magic, at least show that it has disadvantages, too, and don’t just replicate every piece of technology in a magical society, or vice versa.
This isn’t rocket science, people, it’s plausible, non-generic fantasy.