Ian, having an XK-1 low and any other keyboard high is FUN! They need to be really close together if the top one is an arranger, or you have to have an easily operated midi set-up to switch the bottom one from controller to organ.
I often take an old XB-2 to use on the bottom, and a little Np-30 for my right hand at 90 degrees.
I really like to use multiple keyboards and frequently add a synth on top. Bought an SH-201 for that purpose and a GW-7 for the right hand, but haven't used them as much for that purpose as I should.
I use a workstation and an old Yamaha tone generator, plus an old Korg Micro Synth believe it or not on lots of jobs where the primary board is an MS-60 or some kind of piano. A Stage Nord is really good as a second unit...great Rhodes and Hammond sounds in a light package (very red, though!).
Knock yourself out!
Russ
[This message has been edited by captain Russ (edited 11-02-2010).]