It isn't really that hard to fix or come up with, tbh. If you are in single-player you don't control your companions in conversations. You can only use their stats when trading or crafting. Somewhat obscure reference: Vampire: The Masquerade: REDEMPTION (not Bloodlines) was like this. You could only choose the dialogue for the main character, but you could use the other companions' stats to barter. They were totally autonomous entities outside of directly controlling them for combat or shopping, or equipping them. They had their own opinions and interjected in dialogue etc. Just how I think the D:OS2 single player is going to work. The companions are going to have their own story and their own personality and when there is drop-in, drop-out multiplayer the second player isn't going to be able to choose dialogue options.

If you are worried about not being able to use their inherent skills/backgrounds, NWN2 has you covered. When you failed a check in conversations, a companion can interject with their own skills (it was programmed so they always succeeded and it made sense) and pass the check for you. The companions in D:OS can use their influence/abilities/backgrounds/connections to help in conversations like in NWN2