I would imagine it would work similar to just playing a single player game and you pick an Origin character instead of an Origin character being a companion, but I can't seem to find any information.
I don't think it would work the same at all because with other players controlling the origin characters they can really screw things up for you. You might want to try keep Wyll from attacking Karlach for example but with two players controlling those characters Karlach might not feel like being friendly or Wyll might decide to attack immediately, etc. It really creates a huge amount of problems. Generally speaking I would treat multiplayer for this game no different than the D&D tabletop game: ONLY play the multiplayer with people you personally know and are good friends with in real life. Never under any circumstance allow someone you don't know to join your party unless a friend of yours vouches for that person and is there in the game session. If you can't have your regular D&D friends in the game with you for whatever reason then treat it as a single player only game. Especially because a lot of people who will play BG3 have never played D&D before and some will just want to join others games to create as much chaos as they can except there is no undo button once they do something really bad in your game. Plus with all the possible permutations in the longer term this has to be a single player game no matter what because you want to be in full control of every choice in the game without anyone there to screw up what you are trying to do or to argue about doing something else you've already seen, etc.
