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

I've checked the raws in /raw/interaction examples.txt, the were-curse definition cannot apply to: vampires, not-living creatures (aka elementals and bronze colossi), supernatural creatures, other were-cursed, and people who were cursed by a mummy. Actually, all curses - were, vampire and mummy - exclude each other. Pity.

        
    [INTERACTION:WEREBEAST_CURSE]
    [I_TARGET:A:CREATURE]
        snip
	    [IT_FORBIDDEN:NOT_LIVING]
    	[IT_FORBIDDEN:SUPERNATURAL]
    	[IT_CANNOT_HAVE_SYNDROME_CLASS:WERECURSE]
    	[IT_CANNOT_HAVE_SYNDROME_CLASS:VAMPCURSE]
    	[IT_CANNOT_HAVE_SYNDROME_CLASS:DISTURBANCE_CURSE]