If you can contract both the lycanthropy disease and vampirism disease before either disease becomes a full-fledged transformation, maybe.
Not sure, though.
I've been wondering about this too. We already know from previous elder scrolls games vampirism has a 3 day incubation period, so if you catch porphyria, the lycanthropy before the three days are up... maybe.
Or if not, I've also heard that werewolves only have 100% resist disease IN werewolf form. Yeah, this sounds like crap to me too, but it's what I've heard and no one's disputed it (but honestly it just doesn't come up)
So if lycanthropy is triggered, more like a spell, and it isn't a constant effect (again, no one has mentioned "lycanthropy" under constant effects) then why would it necissarily be impossible to be both?
I don't want both per-se, just particularily curious