Yeah, SLI won't help at all with that issue. I'd say you've either got another program running in the background that's forcing the window, or you're mistakenly hitting the windows key or alt/tabbing. I used to have Alt bound as my push-to-talk for vent when playing online games, and quite often I would be talking on vent and go to tab target without thinking about it. (easy to do when you have alt bound to a mouse button)
Completely unrelated to your problem adding a second 8800GT provided you have the cash another 8800GT should be pretty much a steal at this point and time. It actually hurts just to see how cheap they really are and the constant reminder of how much I paid for my 2.