If Octamed supports Midi-acting as a slave, it should work via Midi when you set Renoise as Master.
Anyhow, if Octamed accepts receiving a sysex/Midi CC for start/stop, it should work as well. Then you can send a MidiCC/Sysex from Renoise to Octamed for simultanous play.
When only sysex, see here how to send sysex with renoise:
https://forum.renoise.com/t/sending-and-controling-sysex-with-renoise/18611