Issues running a tri-screen set up

So I am trying to run a Tri-screen setup, however it isnt working properly. So first my computers specs.

Video Cards: 2 x SAPPHIRE HD6850 1GB GDDR5 256 -Bit Dual DVI / HDMI v1.4a / DP Core Clock: 775 MHz
Monitors: 2 x 21.5inch ASUS VH222T (Using DVI) and 1 x LG94WT (Using a DVI/HDMI adapator)
OS: Windows 7

So I currenlty have both of the ASUS monitors plugged into the top video card and they work fine. However, when I plug the LG screen in it is dected but wont display anything. To get it to display I need to disable one of my other monitors. Plugging the monitor into the bottome video card, does nothing, the computer doesnt even detect the LG monitor.

Any idea on how to get it working?
Thanks
 
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