I'm not familiar with linux, and maybe you aren't either?
could just be that you're doing something wrong.
Or, more likely, you are using incompatible hardware. This is sometimes hard to figure out, as it depends on your card, motherboard, and OS.
I had a problem with this at my last company. My CPU had an onboard and installed graphics card. both supported only 1 monitor, so in total I had 2 monitor support... or so I thought.
After literally about a week of drivers, updates, and whining, we discovered that the motherboard on my machine was "special" in that it would automatically disable it's native support if there was a graphics card installed. So even though Windows swore it was using the graphics card, nothing happened, because it was on a lower level than windows could detect.