Second Controller, Live use
I lead worship at my church, usually from keyboard and I run reason from my laptop. I have not used two boards with Reason 6.5 yet, though I am in the process of researching this as I would like to be able to have two sets of sounds coming from two boards simultaneously. I'd also like to trigger backing tracks and have a little more tweakability without having to navigate menus or sounds while playing live. I currently use one keyboard (Novation MKII 61 key) and a combinator patch which uses all the sliders on my board. Assigned to each slider on my 14:2 mixer within the combinator is a different sound (i.e. pad, piano, organ, strings, synth lead, etc.) each assigned to the corresponding slider on my keyboard. Anytime I want a sound, I simply slide the fader up to the level in the mix I want it (I could technically add all 9 at once if I wanted). When the song is done I simply fade out using the sliders and pick my next sound. You following so far?
That being said, If what you said is true about being able to use two controllers at once and "locking" one keyboard onto a device... than could you not simply create two separate Combinator devices (as mentioned above) each with a 14:2 mixer inside and all the instruments and effects attached that you would need for each keyboard? Assign keyboard one to Combinator 1 and keyboard 2 to Combinator 2 and map the faders on each keyboard to faders on your mixers (i.e your top keyboard could be all different pad sounds and your bottom keyboard could be more piano and organ sounds, all to your choosing according to your performance).
Again, this is speculation since I do not actually own two keyboards, nor have I tried this, but I do believe this should work based off of what you and others have said. Should meet your needs! Let me know how things work out!
Computer: Mac OSX 10.6.8, 2G RAM
Controller: Novation SL MKII