I use 3d desktop (beryl) over 2 video cards (gtx 460s) and 4 DVI ports (but 3 monitors). I have even a weirder setup. One of my monitors is 3840x2400 and runs of two dual link DVI ports (for the required bandwidth) ran off one card and the other two monitors are ran off the other card.
XGL is the only thing I am aware of that will allow you to do 3d type stuff over multiple video cards.
I am not sure how helpfull it will be to you but you can look at my Xorg.conf as an example:
Here is what the final setup looks like (youtube video). This video is old and was back when I was using a vp2290b (left monitor) and now I am using a T221 that can take a higher refresh rate (internal refresh of 48Hz vs 41hz) and back then I was using two matrox triple head 2 go in order to go from 2x dual link DVI to 4x single link DVI (what the monitor took). The T221 I have now will natively take dual link DVI.
If you are aware of the limitations (with non XGL) I don't see why you couldn't get working what you want.