I am running 12.04 on a Toshiba Portege laptop. The graphics card is an Intel 945GM/GMS. The 2nd monitor is an Asus (Ancor Communications 22"). When I connect the monitor to the computer the monitor is recognized but nothing is displayed. There is only a black screen. The monitor is not receiving any signal from the computer.
Is it Possible that you have a graphics card that only supports one monitor? Or is your laptop limiting the graphics card? Check to see if the monitor will work like you want it to in windows.
try going into 'Setting -> Display' and select the Asus monitor.
After more research I believe this is related to a bug with Intels 945GM chipset.
I also found one post that said they were able to get the monitor to work by: