A point of etiquette:
When I comment on someone’s blog post, my comment is generally addressed to everyoen who is reading. That’s how I understand comments left by other people as well. My requests are that you adopt a respectful and intellectually charitable tone when you comment, and that you address someone else’s idea fairly understood as opposed to a strawman. But adding your opinion to a back-and-forth between commenter X and commenter Y is not disrespectful. Far from it. It is a recognition that someone has said something worthy of comment (even if what you believe the comment deserves is strenuous disagreement).
If a commenter addresses a particular other person, then my understanding is that the named person’s response is solicited, but that does not exclude third parties from jumping in. Blogging is a free-for-all.
So if you see a thread of comments going back and forth between X and Y, and you feel moved to comment on something in that thread, then by all means comment. You aren’t “butting in” and there is no need to apologize for interjecting your thoughts. If you have something of substance to say, then you are contributing.
I realize some people enjoy seeing the back-and-forth between two individuals, and that’s cool, but as far as I’m concerned, there’s no rule that any territory is reserved. I would rather that Readers err on the side of commenting too much rather than on the side of holding their tongues. If your idea does not withstand the barbs and thorns of intellectual combat, so be it; your personal pride should be only minimally at stake. So throw your thoughts into the crucible, should you wish, and add your twopence where you please even if that is into a back-and-forth between two other commenters.