Fun little trick: while in the UV editor you can select all the vertices belonging to the islands you want to align/stack and hit M (Merge) > By distance and adjust the threshold — just like you do in the 3D viewport. Much quicker than individually snapping vertices on top of each other.
I'm not sure which version this was introduced in, but it's available in 2.92.0
Thanks Tristan! I knew that there was some trick for this but it had been so long since I had done it in the 3d view-port, that I had forgotten how to do it!
