If I remember correctly, Divinity: Original Sin (maybe EE only) has an option at character creation to set AI controls, with different personalities to chose from, even one that agrees with you all the time to skip the RPS mini game. Esentially making one of your PCs as an NPC.
I don't know anything about D:OS2 though, but I would be surprised if they left out this option from the sequel.
That was a terrible alternative. They will either always agree with you or always disagree with you. That's not a personality, that's a brainwashed NPC that you have full control over.
Simply put, you can't always have the game you want tweaked down to the tiny last opinioned detail. If you liked those games then go play those games or games inspired by one of them (like Tides of Numenera or Pillars of Eternity). Divinity Original Sin is not one of them.
Frankly, I think you're just looking at it from the wrong perspective. I think this game is ironically better played as a single player game but can thankfully be fun as a co op game.
You're however not playing a single character; you're playing a pair team, so you have two "main" characters. Everyone else does what you want; they have their own personalities and their own way of looking at the world (hence the persuasion mini events).
This allows you to roleplay your duo however you want, which in terms of singleplayer I greatly appreciate and it's a unique story perspective you don't see often. I can RP my characters as a discordant good guy bad girl duo, as best buddies, as reluctant allies, and not have the game decide for me, which is fantastic because in fantasy literature (novels, mostly) the duo team trope is much beloved and entertaining if done well, but you rarely see it in games without the story/devs decided what kind of relationship you'll have with your partner.
So I fully disagree with you in that it feels like singleplayer was tacked on and, to be blunt, see this mostly as nit picking, but I doubt you'll ever see it as anything else (this is not meant as an insult) because it's a perspective thing. So I don't think it's the game's fault, it's just how you see things.
My only suggestion would be to play something else if it really bugs you, otherwise just try to see your two duos as both being your characters and roleplay how you'd like their relationship to be. Might be tough if you like to get into your character, I know, but yeah, that's it.
Maybe in Divinity 2 just play alone with one character and one man army if you can?