How did Quixel bake that second set of UVs in their tutorial?

In this tutorial, Quixel does a really ingenious use of the built in mesh editing tools to use their assets as a modular set.

They put a bunch of static meshes together and then exported that, then baked a second UV set in Rizom it looks like.

Does anyone have any pointers on how to do this properly? I’ve never seen that before but it looks very interesting