having a best friend who could pass as your twin. not necessarily an identical twin, could be fraternal. finishing each others sentences, watching the same tv show at the same time, thinking the exact same thing, having interest in the same things, are all signs of twinship.
Anna: oh my god, i just figured out who sang that song on that commercial!
Molly: your kidding, i was just watching that and thinking about how i was gonna text you!
Anna: we are sooo twinsies! twinship!