I'm having problems with my second monitor with the following GPU:
AMD Radeon HD 7790 OC(using the FOSS driver)
While having both monitors connected, my xrandr output only shows the first:
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1920 x 1080, maximum 1920 x 1080
default connected 1920x1080+0+0 0mm x 0mm
   1920x1080       0.0* 
   1280x1024       0.0  
   1440x900        0.0  
   1280x960        0.0  
   1024x768        0.0  
   800x600         0.0  
   640x480         0.0
The second monitor can be detected using the proprietary driver, but it is really bug and, of course, proprietary.
Is there any way that I can get dual monitor setup to work with the free driver? I couldn't find anyone else having this problem with this particular GPU.(I'm assuming there is a lot of difference with this GPU's drivers compared to the other Radeon ones, since the proprietary driver is the only one not listed as "nXXX Series" but as "7790" in particular(see AMD site, can't post URLs)).