This may or may not help you. If you installed a second video card, you are running Windows, and you don't have a second monitor plugged into that video card, Windows will shut it off for you. Isn't that nice of it to do that for you?
Soooo... either plug a second monitor into that card, or make yourself a dummy plug, a quick google of VGA dummy plug should lead you to the right place. In the meantime, I've found that if you just start the one card mining, then switch the cable over to the second card and start that one mining, this works fine as long as you don't need to restart your computer.