Depending on his hardware two could run just fine at full size. Definitely sounds like a windowed vs. full-screen issue. You might have to live with the borders but try the maximized option in the videos option and if ultramon can move a wow window to your other screen then great otherwise just move it over as a bordered window then maximize using the windows control box on the border.

If you do have performance issues try this from in wow on both windows once they are running and you are logged in.

/console maxfps 60
/console maxfpsbk 20

The maxfps has to be set before the maxfpsbk will take effect. What it does is lower the frames drawn on the window that is not in focus. 60 or whatever your highest refresh rate for your monitors are what you should use for maxfps unless your system is really chugging then go with less like 30.

Play with the maxfpsbk (and maxfps) until you get a good combo for performance on your system.