Originally Posted by MaDDeePee
What about a separate X for this?
(a second xserver layout)
Thank you for that great idea as it got me started in the right direction!
I was able to create a separate ServerLayout in the xorg.conf, then launch a second X session on another virtual terminal (using startx -- :2 -layout layoutname), and launch my beautiful Doom3 on that X, while my gnome session remained unamused on my initial VT. (BTW I'm surprised how little overhead there appears to be with the second X server running.)
The trouble began when I tried to enable multi-GPU support in the new ServerLayout. I tried putting the option in the "Screen" section and then the "Device" section, but whenever I switch between VTs (e.g., when I ended my second X session), my machine got totally borked. The displays were hosed and my keyboard wouldn't do jack. Only option was the reset button.
So then I decided to just get multi-GPU working on my primary layout configuration / VT / gnome session. The Xorg log file indicated that it was enabled, so I think it's working. The nvidia-settings GUI showed both GPUs associated with my X screen (but no options under GPU-1). And after a Doom3 session the temps on both GPUs appeared elevated.
However there was an insane amount of tearing or some kind of artifacts while playing Doom3 (and then, after I exited, in Compiz as well). So I'll have to do some more research on that.
tl;dr version -- multi-gpu is funky; I'm not sure that you can have it simultaneously enabled on one X and disabled on another.