When you assign a hotkey for a potion, it works for both char.
But only the player which potion bar was last selected, will drink a potion (that is when both are selected)
If you have only one selected only they will drink a potion.
Hopes this confuses you more <img src="/ubbthreads/images/graemlins/delight.gif" alt="" />
<img src="/ubbthreads/images/graemlins/alien.gif" alt="" />
