If i switch the monitors its works fine i use 75 ohm transistors
Your link to cyberdruid's post on OCN will work fine for dummy plugs, its what I used, works fine.
As for your problem, I had the same issue... If you're in XP, right click the desktop, properties, settings, then enable extended monitors on the other cards. Once you do this it'll let the miner be available for a guiminer or whatever.
If you're in vista or 7, right click the desktop and hit personalize, then go into display settings, and enable extended monitors. 1 per card if I remember right.
If you right click on my computer, hit manage, and go to device manager, does it show all of the cards under display adapters?