Once you decide which stompbox is the master you'll want to connect the MIDI OUT from the master into the MIDI IN of the slave.
On the master (In your case, ModFactor,) enter System Mode by holding down both the Right Footswitch and the Encoder button.
In System Mode Enter the MIDI menu and PGM XMT. This stands for Program Transmit. Enter this menu and turn it ON.
Now when you change presets on the Master, the slave will change presets as well.