Here, only one has to claim it, but they have to prove it in court. Usually this is usually done by proving they lived together, bought things together, etc. and if there were other elements of a relationship.
I have no idea what happens if two non-romantic roomates buy a couch together, and one of them moves out though.
Presumably they end up on Judge Judy. More seriously, sounds like something that could be handled in small claims court if there's really a dispute about it.
It can't be handled in small claims court because the claims aren't small and exceed the limits of what a small claims court can rule on - I mean, if someone has spent 20 years in that 'common law marriage' then that generally comes to the full extent of divorce proceedings - how to divide up the house, car(s), other property and custody of children.
I should have been more specific. I was referring to the question about what happens when roommates buy a couch together in order to illustrate it's not quite the same.
I have no idea what happens if two non-romantic roomates buy a couch together, and one of them moves out though.