How to sync bellows with Accordions

[Katzenberserker]

In the manual you briefly point out that you should use the same CC volume for left hand and melody (of course). I'm struggeling now to set this up in Ableton. How can I define mod wheel automation in one MIDI channel and mix it with notes played in another channel? Or is my approach bad and there is a better way? What I'd like to avoid is to duplicate the automation curves for the volume.