For BD I generally had both characters selected and directed them as one, so the combat was pretty much the same as in DD. I did separate them for tough fights when a little micro-management was called for. I played a warrior / warrior for most of act 1, then switched to a warrior / archer, with the warrior in the lead exploring, and the archer set to aggressive (to automatically attack opponents in range).
The gameworld opens up in act 2, and if you are playing a mage, that is when more effective and mana efficient spells start appearing (particularly shaman magic).
Beyond Divinity Strategy Guide
