2 copies of a book, different format, same library, merge?

Ok, I could have sworn I'd seen something about this before, but haven't had any luck searching for it, and so far I'm apparently clueless enough that I haven't figured out an easy way to do it.

Let's say I have both a mobi and an epub version of a book in my library, but because of the way I imported them originally, they're separate books. Is there any way to merge those two together easily?

Thanks.