[ubuntu] Problem using two monitors

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http://ubuntuforums.org – With the proprietary driver of my Radeon graphics card activated, I tried to set up a two-monitor display arrangement ("show same image in both monitors" unchecked), but the system gave the following error message: "Required virtual size (2560,1024) does not fit available size, min. 320,200 max. 1600,1600." Curiously, this problem does not occur when the Radeon driver is left unactivated. Is there a way to adjust those parameters that are causing a problem, so that I could set up a two-monitor desktop? (Hardware)