The method of setting hotkeys is pretty much the same as in DD, but the interface has changed a bit.
To the immediate right of each character portrait (in the top right corner of the screen) there is a column of 3 icons, for the primary skill (left hand graphic / left click trigger), secondary skill (right hand graphic / right click trigger) and potions (bottle graphic). To assign a hotkey to a spell, talent or potion you click on the corresponding icon, then in the menu that appears hover the cursor over the desired icon and press the function key you want to assign to it.
If you use a potion hotkey it will only have the lead character use a potion, so if both are taking damage you might have to pause, switch characters an use another potion.
If you right click on a character's portrait you can set them to aggressive, so they will automatically attack if an opponent is in sight (though the lead character always remains under your control). This works great if your secondary character is an archer, but at least near the start of the game you probably wouldn't want a melee character rushing off after every opponent.
I kept playing my hero as a crossbow archer for most of the game, then switched to a bow. Playing again I'd make the hero the warrior and have the DK use a bow (his armour goes up with his level, so I could neglect strength requirements and focus more on agility).
See the math appendix of the
Beyond Divinity Strategy Guide.