I don't have any official expertise on this but I think in order to really reap the benefits of dual GPU's an application has to be written to take advantage of it. I think that's why some games have "SLI certified" or "Crossfire Certified" types of notations on them. I would be surprised if Orbiter benefited from it, even with the DX9 client. Some games, if we want to call simulations like Orbiter games, see no benefit or even perform worse with dual graphics cards. Microsoft Flight Simulator 2004 and FSX are examples of that (unless FSX is running at super massive resolutions from what I understand), they are far more CPU bound than GPU.
I have dual ATI cards in my laptop, and I tried crossfire with Orbiter 2010. It stuttered noticeably and I had a bunch of graphical artifacts. I Disabled the second card and all was well, granted this was with the standard DX7 graphics. I have not yet tried either OGLA or the DX9 client. I think dual cards really show their muscle when you've got a lot of advanced shaders and mega anti-aliasing and stuff like that going on. Not being familiar with the newer graphics clients, I don't think Orbiter uses much of that kind of stuff. Not to the extent that you would need two cards anyway. I would think that Orbiter would benefit more from CPU and RAM. If you play a lot of modern first person shooters using the latest graphics rendering "eye-candy", then a second GPU might be worthwhile especially if the one you have isn't really high end (they have to be matching cards though). But for Orbiter or MSFS, you're probably not going to see much benefit from it. I personally would go for a decent single card but focus more on a good CPU and a good chunk of memory. I only have the two cards in my laptop because I got a good deal on it (and they weren't the latest card available at the time I got it, otherwise I would have happily taken a single card of the latest GPU). So far I haven't played anything though that really seemed to benefit from it (in other words, I don't play Crysis).
