I'm a fairly confrontational person in cases like this, so if it were me, I'd probably bring it up and start a conversation about what you're seeing/hearing/feeling when it comes to their behaviour toward each other. Get it all out in the open. If things go from bad to worse, walk away for a while if possible, or put as much distance between you and them as you can. Either it'll get better, or not, but you are involved, and you probably do have every right to say, "hey, guys, I'm here, and I'm beginning to feel like an interloper who's futzing up what I thought was a damned good thing between you two, and feeling this way, is WAY UNACCEPTABLE to me. any suggestions?" And then just see what the hell happens and play it by ear.