You have to have one plug free to hook the Y to, don't you? So you're taking one and making two. That connector would hook to your existing wires just far enough upstream from your one free connector to hook to the other drive. But if the piece of wire your one free connector is on isn't quite long enough for that, then go with the Y. But file that type of connector in your data banks as it may come in handy some day.
.bh.