What I'm after is to divide different ranges of keys to different instruments. So like have notes 0-64 go to instrument 1 which is the "low" instrument and 65-127 go to instrument 2 which is the "high" instrument.

No need to use scripting to achieve that. Each Kontakt instrument can be set to respond to a specific range of note or velocity values. Click on the wrench button, then "instrument options". Go to the Controller tab and adjust the key range.

Thanks, I'll give that a shot. Do you know if it does crossfades too?

Don't know about crossfades, but I'd guess not. All that does is tell each instrument which MIDI notes to respond to.
 
One technique for making gentle transitions between layered instruments is to set them up so that there's maybe a one octave overlap in the middle, but to make the overlapped portion play only at lower velocities.  Indiginus' Solid State Symphony is an example of a commercial product that does this especially well, although it uses sample groups instead of separate instruments (but the principle is the same).

think the latter part of the video has a good option.
 
https://www.youtube.com/watch?v=cED7pkZ5gAU
 
I use kontakt, but I find I use TAL a more solely for speed.
 
although another DAW I use can do what you request.. but it's using 2 instances+ of kontakt.. 

This is a specific tutorial for Sonar related to Kontakt. It's helpful!

https://www.youtube.com/watch?v=1u6dWL-32jI