Headless server with multiple gpus

Hi, I have been following in the past the following instructions to run the simulator on an amazon instance or a headless server. While Xorg works great for a single gpu, I would like to be able to make use of machine with multiple gpu cores to run multiple instances at once. By opening multiple Xorgs with different config files, I can run different X displays, each dedicated to a single gpu. However, when running the simulator on each display only the first one can run properly, the rest do not take any gpu.
Is there any way to run multiple simulators, each using a separate gpu?