I will layer clavinette, b-3, Rhodes and/or acoustic piano tracks together.  
I hear all the parts on the record in my head. 
How do you fit that onto your master? 
First you have to play parts that complement each other.  This takes a few passes.  My acoustic piano parts are simple, but because I voice differently than the norm my parts sound difficult to play.  I can play in different styles.  Remember rule three KISS.  
If you can pull off Rick Wakeman level stuff good for you, but it’s not necessary if there are three or four keyboards competing for space.  
Drums and bass down the center.  Keyboard on either side with some 20 degrees between their lanes.  
Pan the keys into their own lanes.  
EQ the crap out of the keys but leave the best sounding frequencies for each keyboard room to shine. 
Play b-3 parts on the B-3. 
Piano and organ are not the same techniques.  
Proprietary info alert!  
Sum the keyboards on a master track.  Your ear will hear them as a unit.  This might be a sauce track added to sweeten, improve dynamics and add reverb or other spatial effects to your mix.  
Always feature the most interesting part playing.  
On the TRex tune Bang A Gong, Elton John is playing piano.  Wakeman was called in to sweeten the song, because it needed something.  
RW added the glissando lick to the tune, and it was a hit.  KISS even if you’re a RW protege.