I’m still a bit confused as to what you are trying to accomplish. Do you want both boards to play the same sound like layers, or do you want each board to play it’s own sound? I use multiple instances of Xpand2 in my songs and setlists. I created racks for each type of instrument I use in Xpand2. Ex: Brass, strings, pads, leads, basses. Each rack has all the presets set to sounds I use from that grouping. I can use either keyboard to trigger any rack or preset I choose. If I want layers, I just route one of the keyboards to multiple racks. If I need to play different parts I can split either board and route each split to a different rack.
It really doesn’t matter which board is the trigger or which rack receives the MIDI. It can all be done with C3.