Hi,
You don't need such cable, although you can use it if you have one. If receiver has only mono SUB output (as majority of receivers have), you just connect it to the Left socket on the Sub and it will work.

Using the Y cable would probably cause that you will fed the same signal to both left and right sockets, but half the intensity (you would divide the output from receiver by 2). So in the end the Sub would receive exactly the same signal as when using normal straight cable.