Journey’s multiplayer system is hidden inside a single-player presentation: other players appear as robed figures without usernames or communication options. The only communication available is a chime. Players cannot type, cannot voice chat, cannot know who they are with.
The two-player maximum per instance and the silent communication were designed by thatgamecompany around a specific hypothesis: that removing all linguistic communication would produce a purer form of cooperation than text or voice could. Players who met a companion and stayed together for the full game’s length were doing something without a framework for describing it.
The credits list companions by their PlayStation Network IDs without distinguishing them from single-player completions. Players who wanted to find the person they had journeyed with had to search by approximate completion time and cross-reference community forums — a process that worked often enough to produce documented reunions.
thatgamecompany confirmed that some companion pairs have played together in every subsequent Journey replay since 2012 — strangers who became regular companions through a game designed to make meeting strangers feel like reunion.

On my first run and now I have to go back and look for this. Worth the detour.
Found this by accident on my third run. Came here to understand what I was looking at. Great write-up.