Agreed. Even though Michael was unique, and in every other case you'd DIE if bitten by both..I don't see why a hybrid couldn't happen in Skyrim. You're playing as a Dragonborn after all, which makes you unique as well.
Unless I'm mistaken, vampires and werewolves are both immune to disease, and given that vampirism and lycanism both are transmitted via a disease, would that not dictate we're "immune" to the other side's bites in the Elder Scrolls?
Otherwise, like you said, I like how Underworld did it: You die.