I'm using a MacPro with Dual AMD FirePro D500 GPU's (3072mb total). Because your software does not provide access to the second GPU, I can run only half of the computers processing power. I'd really like to see an option in your System Settings menu to flag the second GPU for use.
I use this machine for Video editing in addition to high resolution 3D rendering, and animation. X-plane is a wonderful diversion from these tools, yet they make use of dual graphics processing power.