I'm not sure you can use both at the same time. You would have to buy the same card and SLI them, pricey, I know, but I think its the only way. SLI is so inflexible.
I've never noticed a perf drop going from 2 to 1 monitor, I don't think its a problem, or if it is it would be on the order of 1-2%, and so not worth it. And I'm not sure how/if you can set up two cards.