If you are any good at configuring hardware, look around online for a box called an MX-8. It is a MIDI patch bay that holds 50 presets and you can configure setups that include what controls what and send patch changes to each keyboard when you call up a configuration. You can store 50 setups.

This MAY be overkill for what you are doing, and that brings up the question of whether it's too much trouble to press the correct sound preset on only 2 keyboards before you start the song.

When I used one of these I had 3 keyboards and 3 modules, and the master/slave configuration changed with every song, sometimes becoming master/slave/slave, etc... I ended up with a total of 27 different configurations I used that send various patch changes as well as did the MIDI routing. Nice box. A LITTLE confusing to set up at first but in my situation at least, it was much better than pressing buttons on 6 keyboards, setting MIDI channels to know what controlled what.... in a band that played sets non-stop I didn't have the luxury of time. I also controlled the thing from a touchpad that looks like a calculator, so when I needed setup 3 for the 4 Tops songs, I'd press 3 ==> ENTER and everything would be changed to the right configuration. beautiful box, but for that many configurations, it took me like 3 weeks to sit and thing about every song, where was organ, where was piano, where was horns..... and then actually map the thing out.