You simply start a new game as a split-screen game, both of you create your characters at the same time, you go through the intro cinematic etc. and then you disconnect the second controller – both characters are now controlled by player 1 but they both function as player characters. We already tested romance, I successfully romanced one character and my girlfriend another – it tracks approval etc. based on who's the active character that initiated a dialogue etc.
You can obviously use any older split-screen save as well, just load in without Player 2 joining.
It's not perfect and sometimes I wish my girlfriend wasn't looting every god damned barrel... But being able to enjoy combat like in Divinity and never miss a dialogue is worth it for us.