Lego,
It's not just potions, but everything - offensive spells, defensive spells, potions, BF entry (yep you even have to switch leader to enter/leave the BF with a hotkey), ...
It should be possible to hotkey, select target (if any) switch leader, hotkey, select target... at present if you try that you will likely be disappointed, frustrated, confused and dead.
Seems like the DD single character system was implemented for two characters in a rush. Hence the desperate need to patch this.
Cheers