Are you able (and will the CS15 do this), to ‘sync’ osc 2 on the Yamaha to osc 1? If so, then the A4 triggering Yamaha should by triggering osc 1 on Yamaha, since it’s sync’d or ‘slaved’ (again, if the CS15 lets you), then your detuned osc 1/2 should be heard when A4 Trigs it. As someone mentioned, the early Korg/Roland/Yamaha analogs used the Hz/Oct and S-Trig…it all gets weird with Gate In/Out…so experiment. The Yamaha manual will definitely be more informative than the A4 manual is on this topic, and then there’s been a recent update I have not done yet concerning ALL the config of CV.
Back to the monster…finally am digging A4 but since I use all hardware keys and rack modules and record audio into DAW, I have great anticipation for overbridge.