Good news and bad news -
The bad news is that I don't think this is a good combination to cook at the same time. Not only do chicken and brisket need to be cooked at different temperatures, you do not want the chicken drippings to contaminate the brisket, either. To keep the latter from happening, you would need to place the brisket on the top grate (higher temperature) and, unless they are very small, would have difficulty getting the chickens to fit on the bottom grate. (I have cooked beer-butt chicken on the bottom grate but they have always touched the top grate.)
The good news is that I think this is a perfect excuse for a second WSM! /infopop/emoticons/icon_wink.gif