Dual 5870s POSTing but not Displaying

Hi all,

I have 2 ATI Radeon 5870s in my machine. If I place one card in either PCI slot it works fine, and displays on all the monitors plugged into the card (2xDVI 1xDPort->DVI). However if I place the second card and boot no output is displayed on the screens, but the system still posts and boots appropriately.

Ive reinstalled windows (was time for a refresh anyway), and installed the drivers again (latest drivers, what I was using before when it all worked fine), however this does not fix the problem. I have reset the BIOS and checked for any settings related to GPUs in there and there are none.

Both cards appear to work fine when they are in the system independently, it is when they are both in simultaneously that this problem occurs.

Any help would be greatly appreciated