The nature of the wounds, the not submissive character of Meredith who was trained in self-defense, and the fact that she she was dressed, proves that the assailant couldn't be just one. Actions, like tearing and cutting off the bra, needed the use of both hands. So if someone was cutting the bra someone else should have been keeping her still.
While Rudi was trying to undress her, Raffaele grabbed the bra, then he pulled his knife and cut it off. Meredith kept fighting so he ended up with stabbing her on the right side.