Running two monitors off of one gpu creates a hardly noticeable difference in performance and you cant use integrated and gpu graphics at the same time. So running both off one gpu is the only viable option and wont cause any performance issues. Also it wont cut the performance.
Agree with the above. I run three 27" monitors off all three of my rigs and there is no performance problems with any of them. I play HD movies on my 50" Plasma and game on one of my 27" monitors with no problems at all. You would be fine running two monitors off of one GPU with no problems.