Lol, restrained to just 2 keyboards, huh? :p
Well I figure because of all the Piano stuff in the Kronos, I'd definitely have a weighted key Kronos. I don't like having two weighted keyboards; I like having the difference between the keys on each keyboard.
It would be either an Analog-style synthesizer, or some sort of Organ. If I were to choose a Synthesizer... I want to say a Moog Voyager XL, but it's not very versatile. For a synthesizer to go with the Kronos, I'd probably do a rig of several synthesizers MIDI'd together. From what I've used, I'd go with a Roland JP-8000, MIDI'd to a Moog Voyager Rackmount, Korg Radias and a Dave Smith Tetra. I do too much layering for my own good.

If my first Kronos is not weighted though, I'm really not sure now that I think about it. I could say the XF Motif... or I could also say a Second Kronos haha.
Oh, does a Moog Taurus count?
In truth though, I'm a 3-Keyboard Minimum person. I really try my hardest to not have to switch effects live.
- Korg Kronos 88 - Acoustic and Electric pianos, Orchestral and Organic sounds, some synthesized sounds.
- Korg Kronos 61 - More Electric Pianos, Organs, most synthesized sounds, Multi-Dimensional layers, samples, whatever the first Kronos doesn't want to do.
- Roland JP-8000 - Analog-style Synthesizer sounds, MIDI'd with several other synthesizer sound modules, and a computer running Spectrasonics' "Omnisphere" program. (if a laptop can even run Omnisphere :p) The JP-8000 serves as the master controller.
I do a lot of changing of sounds live and I do a lot of using full range of the keyboard with what I'm doing. I write all my keyboard parts to my songs based on what I'd be able to do with my keyboard setup that I have now. (The above is not my setup, just to be clear :p)