I've just installed Ubuntu 14.04 (first time using it so I am pretty clueless) and I can't get my two monitors to be detected. When I go to the settings it detects one of them but it is recognised as a built in display and it doesn't let me toggle that off. Sorry if this is a stupid question but I can't find an answer. Thanks for any help in advance.

My graphics card is a Radeon 7870 HD Tahitin Edition.
My displays are mirrored and set to the same resolution.