This has come up a few times on the boards.

The solution with an ATI card is to open up the catalyst control center, go to the "displays manager" and right click on your second monitor. Choose "Stretch Display onto second monitor"

This is equivalent to enabling Horizontal Span on an nvidia card.

Instead of 2 independent displays you will end up with effectively 1 display with a large resolution like 2560x1024. This will solve your fps drop on the second monitor problem