Two GPUs - double the speed?
X-Plane unfortunately does not support more than one GPU in a single computer. Using more than one GPU, via SLI or Crossfire, can actually slow the simulator down and is not recommended.
If you need to expand the number of monitors run by X-Plane but you are having issues with performance, we recommend networking a second PC to the master machine. You can find some more about how to do this here.