Sport! Either watching an event or as players — possibly in uniform/athletic gear where the social differences between them wouldn't be apparent or important.

Anything skill-based and competitive would be cool, like, Sparks Fuckin Fly At The Scrabble Tournament

Or something online? Where they're always Those Two Assholes having arguments in wiki talk pages or subreddits for [specific subject], and they run into each other at a [specific subject] TED talk, all "YOU! I'd recognise those shitty opinions anywhere!!!"