I'm working on a collaborative project. I've got a dungeon, my friend has some NPC designs, and I'd like to use his faces.

How do we get the faces out of his esp and into mine?

I tried loading both lots, then attaching all the references from my NPCs onto his and deleting mine, but that crashed the CK.

In fact, I'm interested in how to solve this problem in general. How the hell does anyone collaborate when so much of the work takes place in a monolithic IDE?

...for now, I do this with TESsnip from the Fallout Mod Manager.
With that tool you can copy & paste or move records between your ESM's / ESP's.

OK, thanks. That sounds doable. Cheers