They could just load up their server and go onto gm_construct. They could do stuff like make ragdoll poses, try to scare each other, make machines, and all of that stuff. When they get bored of that, they can go onto multiple servers and such.

Would be interesting to see, hm?

Well, they can do other stuff. They don't have to do JUST that. Its gmod. The game spans as far as the both of them want to.