LordCrash, this is a problem that has already been solved many times in RPGs made by companies like Bioware, based on testing for companions. Even your example requires testing for companons present, so I'm not sure why you see this as hard.
Bioware did this from the start. Their focus in the campaign was never coop MP but SP, in the tradition of BG (main character speaks for the whole group with AI-controlled companion interactions). For example in a Bioware game you can never split up your group. Your group is always one entity.
But Larian had a different approach in DOS 2 with their two-player setting and that's even bigger now with the origin stuff and the 4-player setting. The whole dialogue system is built to be played with four human beings while every player can do whatever their want, not only in the overall gameplay but also in the narrative exploration and in dialogues. So in Divinity every character can initiate dialogues for himself, not the whole group. That's a huge difference to how Dragon Age or Baldurs Gate were set up.
Short: In a Bioware game all the dialogue is set up around the main character. Everything is targeted at him and the rare occasions in which a companion automatically takes over are very rare and manually crafted. In Divinity all the dialogue is set up around every possible character. Only very few lines (origin story) are specifically targeted at him and almost every dialogue in the game has different tags and options for all the different characters to answer.
You really don't see the massive difference here?
On a basic level, you could just limit it to character/origin tags, because they're the only ones that should be companions anyway, and then you just refer to the tags.
That was the idea from the very beginning anyway.
In the rare cases where both Sebille and Red Prince have an interjection, you can give each response a priority and choose one. If somehow you end up with 2 Sebilles with equal priority then you just flip a coin.
Rare cases? Pretty much every dialogue in the game has origin/race tags for every single character.
They are already spending a lot of time actually writing the dialogue and putting it in the game. It's not much extra work to set a flag to indicate that a particular response is prioritised as an interjection.
What does "prioritised" mean? If every four party characters have two individual tags per dialogue (which is pretty normal right now), who is allowed to speak at which specific time in dialogue? Only one? All of them? In which order? Right after another?
And the very idea of the dialogue system in DOS 2 is that nobody is prioritised! Every character is equally important in the narrative. Every character can be "the chosen one". That's the very idea of coop MP and character development in DOS 2 because every character is a possible main character. DOS 2 was never set up around traditional companions, like e.g. in Bioware games. Everybody is a possible main character, so every dialogue line is equally important (with the one exclusion of possible origin stories/quests).
So yeah, I do think that such a system would require not only a lot of extra work but even a shift in narrative vision.