Hi Lee,
in my set up no.
I’m really just replacing my sx keybed with the p121 keybed so that I can play weighted keys instead of synth keys.
Whatever split is assigned on the sx900, my p121 uses. I’m not using any p121 sounds in my setup. It’s really just giving me an sx with 73 keys.

I’ve never been an organ player , so stacked keyboard set up ( to try and emulate an organ) doesn’t work for me.

Mine is an L shape configuration. I have a set of monitor speakers in front of my p121. I have my sx to the side ( plugged into the monitors) .

So when I want to play solo piano, I just turn on the p121.
When I want to play sx900 with 73 weighted keys , I turn on sx900 and p121 and monitor speakers. I use a pedal to walk me thru registrations on the sx900.
If I just want to play SX900 in normal mode, I just switch it on.

The p121 doesn’t have dedicated midi sockets, so at the moment , I just use my laptop as a midi interface between the sx 900 and p121.

Dave will have to tell you how his setup works.