I have 2 ATI Radeon HD 5450 video cards installed on my desktop. I am running Windows7 64bit OS.
Currently I have 2 monitors hooked up and enabled as my displays. One monitor hooked up to each video output. Currently each video card has a DMS-59 output and I have a dual/splitter cable hooked up to each with DVI-D outputs.
I can only get 2 displays to visually work and have an extended desktop when I have one monitor hooked up to each video card and thus each splitter is just using one output of the two that are there.
When I try to hook up a 3rd monitor, (1 splitter has 2, other splitter only has one monitor) .... Windows recognizes it and I can see it identified via the Control Panel Gui. But after "applying" the extended desktop configuration with the GUI... the monitor stays blank. (I have checked on the 3rd monitor that it is looking for the DVI-D output and not the VGA analog one).
Also if I only try to hook up 2 monitors via one Video card and the DVI-D splitter, I can only get one monitor to display... even though Windows recognizes 2 via the Control Panel GUI.
Had a similar issue on a workstation. I removed one of the cards, hooked up the 2 monitors to the single card and both worked on the next boot. Then reinstalled the second card, plugged in third monitor and everything was working.
Not really a solution as to why you're having the issue, but could resolve the problem.
Good luck & update your drivers if you haven't already. Oh, and your cards are probably OEM parts so you might want to try the drivers from the OEM first vs. from the AMD site.