I needed to deal with a similar setup (multiple screens) and as far as I understand when working with multiple screens there is still no answer to hardware acceleration, and Xinerama is needed for sure.
But if I were you, I would try to figure out if OGL NEEDS hardware acceleration or just supports it, and try to figure out if your code uses hardware acceleration or not.
It could be you can get away with 3D OGL rendering with out the need to use hardware acceleration, but I don't know much about that subject.