In a cave known on some planets as "Altamira", where there are prehistoric drawings on the walls across many worlds, there is an event.
It'd be sort of hard to miss. There's a lightshow, some harmless radiation spikes, a resonant high-pitched noise that lasts for several hours and is really annoying for miles around, basically it was thoroughly obvious. The interesting thing was that the cave then proved to exist in several universes as a single cave where previously it was one cave to a customer.
We will elide here the frantic linguistic nerdery, the protocols necessary to ensure that no one brought a flu home, the physics experiments for determining how things split back into their own universes (you can go to someone else's, if you hold their hand and let them precede you out of the cave; unattended objects get a little squirrely but not so much that one can't run cables), the security arrangements each world undertook at the aperture, and the installation of conference furniture and a water cooler and everybody's respective Internet access. Instead we will open on the Summit: contingents of diplomats and whatever auxiliary personnel each world found meet, assembling in the cave. (Please don't touch the paintings, some cultures care a lot about those.)