As far as I know you can use WoWMaximizer to send the second client to the secend screen.

If you use 2 identical classes with identical keybindings you can use Keyclone to send keystrokes to multiple clients. See: http://www.dual-boxing.com/forums2/viewtopic.php?t=883

The author of Keyclone also frequents these forums and is a helpful guy. He's willing to help you should you come across any trouble.