Wiring Diagram for Split Micro-USB Cable?
This does not work. You will need a USB hub.

The USB Y cables you have seen use two USB Type A male plugs to one USB B type plug. Only the power lines are split to the extra plug.
This is not conform the USB specification. But it is a small hack to have more than the standard 2.5 Watts of power available, it is used for 2.5" hard disks that can't spin-up with just one plugs worth of power.