cosmowe - thanks so much for the quick reply! Sorry I didn't see it sooner, also thanks for the screenshots - very helpful!
Unfortunately I think in my enthusiasm about Resolume, I didn't as clearly detail - my collaborator doesn't have Resolume, and would be performing some video game/art conent (I think from Unreal). Is it possible for them to just line out an HDMI cable into some recommended capture device, that from their computer would assume it's just a second monitor, and I can use Resolume to use it as input, which I can then do overlays and switch to my video materials back and forth?
I feel like the BlackMagic Ultrastudio Express, or maybe some newer capture card that takes USB-C directly might be in down the right path, but not sure on how to best configure this. Thanks again in advance!