Can a person truly belong to TWO separate religions at the same time?

Does one have to subscribe to a particular faith to become a member of a congregation? With only a handful of exceptions, I can't think of any religious group that demands no one but them can enter their places of worship. (Latter Day Saints come to mind, but I'm not even sure if that's only portions of their temples or the entire structures.) I'm Jewish and I've sung in masses. I've been to many church services of all denominations, actually, and the only thing I've refrained from doing was taking communion. I've never actually joined a church, but for that matter, with the exception of being a member with my parents when I was a kid, I've never joined a synagogue either! But activities, and services, and being a part of the community? I can't see why anyone would be refused that at any house of worship.

It wouldn't be officially recognized, but many churches will 'look the other way', so to speak.