I want to run dual monitors and find that I can do so if I select the resolution to be 800 x 600 for each, since the sum of dimensions when not mirrored is 1600 x 600 both landscape or 1200 x 800 both portrait.
The trouble is that the natural resolution of my monitors is much more than 800 x 600, in fact it is 1440 x 900. Summing in either direction exceeds the 1600 x 1600 virtual space. Is this a limitation I can improve by using a different video card (and matching driver)?
Edit: I am using a Radeon HD 5450 and the latest (Dec 2011) Catalyst driver for x86-64 architecture on Ubuntu 11.10. I did not write a custom xorg.conf.