That is the general issue with 3d MMO type games. Because the camera views and perspective of each character in game is never exactly the same, then sending movement/mouse clicks to all screens will generally make your characters spread apart or move to different locations.

It works better with some other games like Diablo3, because the perspective of each player is fixed and the same, although even with that, the characters can still get out of sync in positioning, with stuns and kickbacks, and even game instance lag.

This is why most multiboxers of MMO's will use in game features like Follow to move the "slave" characters, and Assist features to get "slaves" to help the main toon. Broadcasting is normally only applied to getting skills to fire in game on all toons, or interaction with 2d items, like menus.