Many years ago when I bought my first keyboard, I added a second keyboard, a Yamaha Synth. I was at Sam Ash’s office talking to Jerry Ash, the owner. I knew him personally; I taught his son guitar. Jerry Ash was talking about the new type of keyboard they just received; it was a Yamaha Synth, to my knowledge it was the first of its kind.
I bought a book, “How to work with Midi”. And then simplified what I learned.
1-When someone speaks, the listener must be tuned to the same station (Midi track) to hear him. Master keyboard; piano on Midi track #1, Synth keyboard must be set on track #1 the number the represents piano on the list of instrument sound and Midi numbers. Simple?
Yes it is, if you stop right there. There are 16 Midi tracks, and within each track there are more tracks, areas to add what you want. Now it becomes a bit difficult. My first attempt was to set the master keyboard to piano, and the Synth. to strings. I used a volume control between both keyboards. I was able to add strings and the volume to my piano.
I hope this helps, John C.