I’m new to the Octatrack so take my thoughts with a grain of salt, but I have a somewhat similar set up–Octa, A4, and Prophet 08. I have the Prophet hooked up to the MIDI IN/OUT of the Octa and I have MIDI THRU from the Prophet to the A4. This way the Prophet and the A4 both receive MIDI notes/clock from the Octa and the A4 can also be set to receive Start/Stop. I can enter note data into the Octa using the Prophet and from there route it back to the Prophet or the A4. I can also just use the A4’s internal sequencer with the Octa providing clock and start/stop. Hope this helps!