I was under the impression that you could not use a (for instance) 1.0 and 1.3 card simultaneously in the same app. I am not quite sure where I got that from though…
I did just try this on one of my systems - splitting a single computation across a C1060 and a Quadro FX 5600. The results appear to be correct, and the computation took less time than simply using either card.
Am I just mistaken in what I remembered, or is there some reason I should not do this?