I know I've brought it up before, but this is reminding me of the encounter with the Gith if Lae'zel is in your party. I think she should be the one Kithrak Voss addresses, and I don't think someone should be able to sub in for her possible deception check.
But there are certain other checks have clear justifications for other characters to pop in and comment, Shadowheart and Gale in the catacombs for instance. In fact having characters interrupt your characters musings to comment on something is an underutilized way of giving our companions so more opportunities to be fleshed out.
Yes, I agree that for certain conversations or checks the character speaking and doing checks should be locked in, or at least should involve some special option to accomplish and explain a hand off, primarily when an NPC is very definitely interacting with a specific party member rather than the party as a whole. This does complicate things a little, but shouldn’t affect too many different encounters so hopefully should be manageable.
And I’d definitely agree that there are far too few occasions when our companions interject in conversation or as we wander the world, and it’s annoying that when more than one companion has something to say they seem to override each other so we only hear from one. Having a companion interject might also sometimes be used as a prompt to single players to consider tagging them in for dialogue if that were an option.