Some gamers like to use a second display for reasons but test after test show that that second display does take away some GPU power.
I used USERBENCHMARK to show the impact.
Two Monitors -> http://www.userbenchmark.com/UserRun/10848149
One Monitor --> http://www.userbenchmark.com/UserRun/10848394
Here's the before and after results:
There's more you can do. I like to read a Speccy report and see what is going on but when a gamer is reaching for those last frames, here's an easy one.