Recently I did a two-fer to see if it would work:
(view image for full 1920x1080 res)
It did. Both games ran at full speed as well. (To note, the graphics settings on both are up as high as I can put them. Borderlands 2 has full PhysX as well as ini tweaks. Crysis has 4xAA as well as everything being set to Very High. No ini tweaks as I just installed it. The resolutions are 1024x768 ... so I can see both of them. I can't really increase the resolution and still check if they still render while on the other game if I can't see both windows.)
I was actually really surprised this worked. I was also alt-tabbing between Burnout Paradise and Split Second ... it made the cars in Burnout look weird by comparison. (Average swap time is a couple seconds, including resuming from auto-pause)
Anyway, I was pretty shocked to see it not only worked ... but the other game (well, if it didn't auto-pause when losing focus) would actually continue to render like normal at normal speed.
I'm going to look into a way to do it with independent displays to see how this functions at 1080p resolutions.
As for a "real" screen shots ... here's a ENB testing I did in GTA4 (It's CryENB 9 ... 10 has some annoying as hell depth of field that makes it so you can't see anything more than 3 feet in front of you, and IcEnhancer is somewhat bloated for what you get ... also I want to play online.)
And here's where I first learned the new setup could play Borderlands 2 with full PhysX running: