Is this set up appropriate?
You need:


two MCUs that can act as USB devices (one per slave),
some way of getting the commands from the master to the MCUs


That means two Arduino Dues (or Arduino Leonardos, or SparkFun ATmega32U4 breakout boards, etc.) and one USB device of some sort that you can talk to and have send commands to the MCUs.

And perhaps something to transport the commands from the master to the slaves, but you need to think about the programming for that first since the transport and the programming will be very closely connected.