Actually now have my sx stacked on top of p121, midi connection via my laptop.
Decided to go for the simplest midi setup I could for “arranger mode” which is simply to use the sx900 for everything and just use the p121 for its weighted keys
For Solo piano, I use the p121. Sx gets switched off.
I also stack it on top, so I use 2 different midi patches - one that triggers the SX from below, and another that disconnects the two, so I can trigger styles from the SX with the LF, while playing full piano from the RH. If I need more variety, I'll write a third that connects the 121 to the SX for a layer voice only, and no arranger stuff.
It's a great set up. Show us how it looks, Rikki!
Here's mine ...