"The first one to act like a shouting dramatic asshole gets asked to leave". Because any functional adult should be able to **if nothing else** say "excuse me" and go to the bathroom/get a drink/whatever else if they're around someone they can't stand. As you say, this day is about you, not their petty shitbaggery, and whoever clearly makes it about them and their pride and their shitty attitude is demonstrating that they really don't care enough about you to just keep a lid on it. And do you really want someone in your life who'll act that way? So tell them both to grow the fuck up and steer clear of each other or you're basically done with them, because why wouldn't you be?