Actually it's pretty common in vampire universes (if I can call it like that XD) It's slightly different depending on an author and the basic idea is Vampires are feeding on humans (duh) but it's not uncommon they feed on each other especially when they're lovers. I saw this motive a few times (and I like it XD)
This author created a universe where vampires (out of respect?) stopped feeding on humans and feed on each other. It makes more sense to me then feeding on human sperm (there was a story like that, like wtf? XD)
I thought there blood has a issue so cant feed on each other.
So Vampires in this feed on each other that is new?