I don't think anything at all will happen. And there is no reason why they should be crossing their ways more often on a US tour. Slayer and Megadeth just recently toured Canada and Australia together and they (Mustaine, King and Lombardo) only bumped in each other for five minutes once and that was colloquial. Everyone is doing their thing...
Machine Head was also part of that Canadian tour and Flynn said, he didn't get to see Mustaine even once.
Everything is totally separated, no matter where they are.
Of course it's business, but does that even matter? It's not like they're pretending to be best friends all of a sudden and they don't have to. Who cares? It's just a show... Hell, I can't stand many of the bands I'm playing with either.
I just want to see it because I like every single band on that bill.