Welcome, OldDude, from another old dude!
Huh, looking at the CDP-220R's manual, I can't see any functions for setting the relative volumes of the main, layer, and split tones, so it appears that there's no way to modify the volume balance between those tones-- at least, not on the keyboard.
Offhand, the only thing that comes to mind is that you could try connecting the CDP-220R to a computer and use a DAW to send MIDI messages to set the Channel Volume for the desired channels-- channel 1 for the main tone, channel 2 for the layer tone, and channel 3 for the split tone. If the keyboard's panel settings don't respond to incoming MIDI signals-- e.g., if sending a Program Change on channel 1 to select a different main tone doesn't actually cause the keyboard to play with a different tone-- then you'll need to set the keyboard's local control to "off" and have the DAW bounce the keyboard's MIDI messages back to the keyboard.
That actually isn't as difficult as it might sound, and would open up some other possibilities as well-- e.g., layering more than two tones together, or creating more than two split areas-- but I don't know how convenient it would be for you to have the keyboard connected to your computer while you're playing it. If it wouldn't be feasible, then you're stuck with whatever functions and controls are built into the keyboard.
