"merge" minecraft worlds

I run a Minecraft server.

I want to duplicate the server, and run two servers now. Server A and Server B.

At the moment, world A and B are identical.

I want to let some people (players A) onto Server A to break and place some blocks.

At the same time, I let other people (players B) onto Server B to break and place some blocks. Players B are nearby to where players A are in Server A, but not so close as to be building in the same blocks themselves.

After both groups of people have finished building and destroying parts of the two different worlds, World A has things in it that World B doesn't, and vice versa.

How can I create world C that has both all of players A's changes and players B's changes?

E.g., if players A built a house in world A, and players B built a house somewhere else in world B, world C should have both these houses.

If they are far enough apart, you can just overwrite the region files of one world with a part of the others. Region files save a part of the world that is 1024x1024 blocks big.
So if for example players A build on X coordinates lower than 2048 and players B on X coordinates higher than 2048, then you can just copy the region files called r.[number ≥2].[number].mca from server B to server A and confirm that you want to overwrite.

Region files are found in
.minecraft/saves/[world name]/region for the overworld,
.minecraft/saves/[world name]/DIM-1/region for the Nether and
.minecraft/saves/[world name]/DIM1/region for the End.