How do I make 2 separate bones both move one other bone? or rather, how do I parent a bone to two other bones so both control it?

I have a bone near my chest that I want both my shoulder bones to be able to move. The problem is when I parent the chest bone to, say, my right shoulder, now my right shoulder moves the bone correctly, but if I try to parent it to the left shoulder it unparents it from the right. I want both shoulders to move the bone