Both DX and OpenGL allow for multi-monitor gaming, but it is in the application that you have limitations.
In other words, the game must support the dual monitors. Also, both monitors have to be connected to the same video card (which I believe is your case)!
Note, this applies to 3d full screen rendering, 2d is not affected by this!
I run dual monitors, but off of 2 video cards. 1 is the Radeon 8500, the other is a Radeon 7000. I had both monitors connected to the 8500, but I had DVI distortion

T'was not a pretty site:Q
Hopefully your dual monitors will not have the same issue as I had (with the DVI)!
Edblor