tons of people running multi gpu's on reduser and other adobe forums....
this forum is just not super active...
there can be little issue's and gotcha's with multi GPU's....especially on speedgrade, but they can be sorted out
I would run your primary monitor on the 670....the secondary as well, that way if SG is only accelerated with 1 GPU at least it is using the fastest one.
Also I had major slowdowns by splitting the speedgrade monitors on separate GPU's...don't do it.