Two of us?

We've heard this a lot, the phrase "there's two of you". Not really, there's one of me & one of her! I know what people mean. We're so much a like and there's always been the "us" factor. Whenever I talk about my birthday, it's "our" birthday. It's never been "mine" really (which is OK.) But still, there's just one of me and one of her!