Interestingly enough I've just been reading through the Apple developer documentation for other reasons and I can across an article discussing this very issue. From what I can tell it should be possible to use 2 graphics cards to render images but it would need to be alternative frames, this would though still increase performance as the cards could queue the next frame up while the other card is displaying its image. It might require some interesting code to get the image from the offline card to the online card though.
Anyway back to the topic in hand, I very much doubt that X-Plane would take advantage of this anyway but it might be interesting to play around with it from an academic perspective.
For those interested:
https://developer.apple.com/mac/library/technotes/tn2008/tn2229.html