Multithreaded Rendering is usually a good thing. It allows the GPU to do more stuff at once. Only if it causes issues would I suggest turning it off.

But, back to your other issue.
If you have something using your 2GB of vram, and you have set the games to run at a low resolution with low in game settings and it has not helped, then maybe whatever your 2 games are, just aren't going to be compatible when attempting to multibox them.
It is fairly rare to find a game that even on it's lowest settings requires more than 1GB of vram (assuming a 50/50 split here), but certainly some newer ones are going that way.

Ideally you would test each of the games and find out how much vram each game will require, and see if the total for both is less than 2GB. If it was the same game, then you do get a little bit of leeway, with caching and reuse of resources, the same game is a little more efficient and multiple copies tend to not require the same multiple of vram. Different games most probably won't benefit from any sharing of resources or caching.