Well consider this: as seen in the 100th episode, they live together, but they divide the house. We can tell they're not a couple because they wouldn't divide the room like that. They're not roommates because if they were, they'd probably have split up by now. They both have differing music tastes, but you can see the mutual respect that they give each other despite this. And they know each other well enough to communicate with few or no words.I have two sisters, and Vinyl and Octie have very similar behavior as well.