I was about to ask if you know of anything that did this (you did).
I’m trying to understand how it could be done without a midi merger or DAW etc.
So you have a controller going midi in to Octa (or Nord), this controls the Octa and also should go to the midi thru, not to midi out. Midi out is data from only the Octa (but with CC connect and being on the appropriate channel it will, first piece of gear Iv’e ever owned that does this :+1: ). So you want the Octa to control your external synths via midi out and the controller to control the Octa and your synths ?
Too many ands in my knowledge of Midi.
Thats what a Midi router is for. Send multiple Ins to multiple Outs.