I don't think this has been tested by
official SoftTH forum members yet, so you're not very likely to find the answer here I'm afraid... (We have a
SoftTH section though, btw) ;)
If I'm not mistaken, it's not that SLI is "not supported", but it won't give you any framerate boost with SoftTH. So you're better off using non-SLI'ed, single-GPU cards. Other than that, each game can behave so differently with SoftTH, that it usually demands quite a little knowledge and patience to make things work as planned - so I don't think topping that with 120 Hz refresh rates, 3DV background app and stuff is going to help. :?
As for 3D Vision on Multi-mon setups, I already made my two-cent statement
over here.