Well, there was once a dual pasthru card that i had.. it was a third card that had two passthru cables, one for each card, then the video out... But that was back in the old days when 3D cards did crappy 3d, so you had a 2d card also...
Anyways, you could do it, just wouldnt recomend it... Would have a lot of monitor switching when changing your mind on what video card to use....