Fullscreen mode with two monitors

For an experiment with two participants, we want to have a single PsychoPy window extend across two monitors. The two monitors are set up under Ubuntu using Compiz to treat them as a single output (as described in full detail here). However, when setting up visual.Window with fullscr set to True, the experiment window is shown on one monitor only, and the other monitor is simply black. Other applications on Ubuntu extend across both screens, so the Compiz setting seems to be working as intended outside of PsychoPy. We tried to follow the steps described here, but setSizePix does not seem to be taking an effect, because PsychoPy shows a warning “User requested fullscreen with size [3080 1200], but screen is actually [1940, 1200]. Using actual size”. Is the Compiz setup somehow incompatible with PsychoPy? Are there possibly other ways to have a window extend across two monitors in fullscreen mode?
Grateful for any pointers or tips.