Having a were-curse prevents the target from collecting any other were-curse, among other things. So... nothing besides normal bite damage.

Having checked the raws in /raw/interaction examples.txt, the were-curse definition cannot apply to: vampires, not-living creatures, supernatural creatures, other were-cursed, and people who were cursed by a mummy.

Actually, all curses - were, vampire and mummy - exclude each other. Pity.