for example; my best friend practices Native American beliefs (her father was apart of the Illinois tribe), whereas I am agnostic. We get along extremely well, and are very respectful of each other's beliefs
and on the same hand, her husband practices some Celtic beliefs, and they have no issues with each other
Well, yes, oftentimes people of differing religious beliefs can coexist. Someone with different religious beliefs from me can still be a very moral person. I may find their religious beliefs to be nonsensical, but as long as they aren't trying to enshrine them in law and aren't bigots we're likely going to get along just fine.
However, I do think there are times where this could cause issues. For example, I don't think I would want to date a devout Christian. Even if I have determined that despite that, they are a good, moral person. The reason being that because I'm a non-believer, if they really loved me, that would weigh on their mind. They would be worried about the fate of me in the afterlife, that I wouldn't be there with them. This could lead to proselytizing, which despite me not being able to blame them when it's that genuine, would create a rift. Also, I just wouldn't like that weighing on their mind even if they didn't express it, because they're going to worry forever.
It's for that reason that I'm not public about being an atheist and my parents don't even know.
(And honestly, if it
wasn't weighing on a devout Christian's mind, could I even trust that they
actually care about me? It's a very bad position to be in, one way or another)