I created two worlds based on Sunset Valley lite. Naturally, the game regards them as the same world, since technically... they are. (Reality is so annoying.) As a result, I can't have both installed at the same time. Is there some way to go into one of the world files and change a name or a line so that they are considered two different worlds?

You should be able to change that in the world's description/name stuff. Right-click on the world name in the very top of the World Layers panel and do "Add/Edit Description"

I already gave them separate descriptions, names, and thumbnails in the add/edit descriptions. I think it's that the Sunset Valley lite is not just a height map, but a world you have to open. So not creating a new file, but opening an old one. Therefore, any changes are just edits from the game's perspective.

You could export the Sunset Valley height map by going to Terrain > Export Height Map (I think?) and then create a new world with that height map instead.

Thanks Lukyzade, that solves the problem for the future. I now have the height maps for Sunset Valley and Riverview. Just wish I could rescue the worlds I already made.