I use one main 88 key weighted piano-type plus a 49-key synth-type controller with some pads on the upper one.
For switching states, my main control is the big red button (What's your preferred method for changing states? - #6 by Torsten) in the middle of my lower piano keyboard. It’s nice and friendly and reacts to a very quick and light touch (but doesn’t get triggered accidentally when just brushing it. With it sitting in the center of the keyboard, it’s easy for me to trigger it with whichever hand is less busy in passing.
Plus, I’ve marked one of the pads on my upper KB in red and tied it to a binding, so this can be used alternatively when my hands are both “upstairs”.
This is my default way, embedded in the background rack. In some (rare) songs, I may be too busy with both hands; for these I usually create a specific binding triggered by the left side of my foot pedal. I try to avoid that, though - I play standing up, and I need at least one foot to be stable and the other focussed on either sustain (piano work) or expression (organ work). Searching around underneath my keyboards for the correct pedal while singing and playing is just too much for my brain, so I usually prefer the red button.
Cheers,
Torsten