2 Keyboards. How to drive the first by playing the second

I have a Nektar controller connected to the MBP (with Mainstage) via USB cable and a Kurzweil PC4 connected to the sound card (Scarlett) with 2 MIDI in and out cables (5 poles).No others connection. In the MS concert I have 1) virtual instruments controlled by both the Nektar and the Kurzweil, 2) external midi tracks to play the Kurz piano. I would like to be able to use the Nektar also at playing some internal sounds of the Kurzweil, but I don't know how to set the MS parameters. What kind of routing? I can not find a suitable combination. It's possible? Thank you

MainStage 3 - MBP M1 Sonoma