I do like the two boyfriend analogy, but with a twist…
I’ve always used a Scientology model to explain the relationship between the Bible and the Book of Mormon. The Bible, for example, would be Dianetics, because Dianetics explains how to reach a state of “clear”. The Book of Mormon, on the other hand, is like Battlefield Earth; you may not be able to reach “clear” by reading Battlefield Earth, but that doesn’t mean it’s not a vital part of Scientology.

Battlefield Earth, if memory serves, is one of Mitt Romney's favorite novels.