Ubuntu 11.10 won't detect my second moniter

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http://askubuntu.com – When Ubuntu boots, the second monitor mirrors my other monitor, but when the login screen pops up, the monitor doesn't get any signal from my computer. When I go to monitor setup, it is not detected. If I unplug my first monitor, the second works fin with Ubuntu. When I tried 11.10 on a flash drive, it worked fine as a dual monitor. The default seems to be VGA over the DVI connection. Help? (HowTos)