Hello, I have been multiboxing for a month now, and still playing around with settings and macros. So i will try my best to help you out on your problem.

From what I read on the post of "mxracer050" on:

How to Box 5 accounts (or 2 or 3 or 4) with ONE computer

he said that:

"Unfortunately, the thing with XP is the Direct X part of it. When you have 2 monitors hooked up and on “Dual View” there is a glitch, and it cuts your FPS down in half. Making it unplayable.

The solution to this is Horizontal Span. Go to your video card settings and change the display to Horizontal Span ( one whole desktop ) This will move your resolution up to 2560 x 1024."

Hopefully this helps. May i also what graphics card you have? I have heard that upgrading to a 8800GT or equal to that card. will help alot on Frame Rates. To read more about screen setup's go to the url I posted above.