I feel this is a decent compromise, but I disagree with LordCrash that it's difficult to have companions interject without the player explicitly asking for it.
It's simply a case where your AI companion makes the choice before you. As an example, a conversation where Sebille would normally have 4 conversation options, including one that she is very passionate about (and is potentially unique to Sebille). When she is your companion, and you reach this point in the conversation, Sebille interjects her favourite response before you are given the option to respond.
The only difference from HauRukh's suggestion is that instead of selecting "Let Sebille answer this", the option would be selected automatically in cases where Sebille is strong-minded enough that she doesn't feel the need to wait for your approval.
It would be even more awesome if in co-op, there were a system in place that allowed co-op partners to interject into each other's conversations under specific circumstances. Once again, providing the sense that both players are participating in the conversation.
