I doubt you're seeing actual SLI behavior with fullscreen windowed.
Multi-GPU setups + windowed games rarely ever work properly and provide really inconsistent results even if they do.
So far as I know there is a tiny handful of games where SLI functions semi-normally windowed, and a grand total of 0 for CrossFire due to obscure, arcane architecture/driver design reasons.
I know... but I had to try at least... Its aggravating that I have this extremely powerful machine that can run games like Cyrsis 2 on Ultra (DX11 patch) + NVCPL tweaks, Mass Effect 1, 2, and 3 (also with NVCPL tweaks), and Dragon Age II (DX11, upgraded texture patch, and NVCPL tweaks) and have a solid 60FPS no stuttering and just smooth wonder bang bang love. y'know? But when I try to run SWTOR ... I'm doing technical stuff to get it to run and look even better than it does while maintaining a messily 60FPS...