With the state that disc is in right now, having 2 discs just isn't a problem. Getting rapture every 12 seconds can happen from shielding a dps on most fights, so fighting over tanks isn't a big deal. Just organize spirit shells and you won't step on each other's toes.
and I agree you should leave the shadow priest be unless he is shielding himself randomly, or doing it just to WS himself to troll you. Body & Soul is still a talent some priests take, so you can't really get mad at them for using it. (Even though feathers is the best talent ever)