MC. I do not know about PA80 + X1, but as a Guide, this is how its done on the Yamaha.

Richard Bower. Yamaha Demonstrator uses a Tyros + PSR290. (Now PSR292) The Tyros (Master) is split last key left. The full length of 290 triggers Tyros ACC & Lower voice. This leave you the full length of Tyros to play right hand melody. No Voices are required from the 290 so you can position it close under the Tyros with just the keys showing,no buttons required.

MIDI SETTING for 2 Yamaha Keyboards

Master Top keyboard. Set Key Split to last key LEFT
Select FUNCTION
Select MIDI
Select ALL PARTS then press EDIT
Use NEXT to select RECEIVE
Set First top Part to CH1 LEFT
Use NEXT to select CHORD DETECT
Tick Channel 1
SAVE INTO USER with a name.
The PSR292 now controls the Tyros ACC & Lower voice over its full length.

Graham UK