So I have given up on the notion that Larian is going to ever do anything about this topic. My wife and I started as a split screen co-op just to have 2 custom characters and we transitioned into a single player campaign like others have already suggested.
1. start split screen with 2 custom characters 2. I turned my controller off 3. turn the controller back on as an assist controller 4. enjoy a completely full screen experience
The only minor con of this approach is that whoever is the assist controller doesn't get free movement of their character at the same time as the main player's. But having tried both this and the split screen co-op, this is such a better experience. There is no split screen during exploration nor battle, and whenever there is a dialogue both of us fully engage in that one dialogue that's happening and make decisions together. It's a shame players have to find a workaround to actively avoid a "feature" that locks players into a worse experience. And also a shame that Larian does not seem to be hearing this issue or just not addressing it at all.
wait a sec, the always merged screen is actually my preferred way to play it, ignoring turning-the-controller-off part, i'm so gonna try this.
