WoW 4.0.1 known issues thread: http://forums.worldofwarcraft.com/th...88059429&sid=1

Starting more than 1 wow.exe simultaneously will cause the applications to deadlock
- If you are multi-boxing, do not use a batch file or other method to launch multiple game clients at once
I'm surprised anyone would see this issue with ISBoxer given that description of the issue, since it will only launch the next wow.exe (or any other game for that matter) once the game window has launched. If it's accurate, you could try increasing the delay between launches in your Character Set, which defaults to 1 second (after the previous game window is up and rendering in direct3d, and has been set into its window layout by ISBoxer).