2x GPU's in SLI better then separate GUI card?

I'm putting together my first "serious" system, but I do a lot more then just color grading. The programs I use daily are Avid MC6, After Effects CS5.5, Resolve, & occasionally Premiere CS5.5, all on Mac OSX. From what I understand if I setup a cheaper GUI Card + a bigger dedicated GPU for Resolve my other programs will only take advantage of my weaker GUI card since thats what my monitor is plugged into. I also read that if you use one card as a GUI & GPU for Resolve it essentially cuts the GPU's performance in half, giving half to the GUI and half to the video processing.

So my question is, would it make sense for me to tie together a pair of GPU's via SLI, like two GTX670's? If two SLI connected GPU's are treated as one, big GPU, then After Effects, etc could use the full power of both cards, & I wouldn't lose any performance in Davinci over a GUI + GPU card since it would essentially be splitting the combined resources of two cards in half.

Sorry if I worded that weird, but does that make any sense at all? I'm wondering if there is something I'm missing here.