Guitar VIs are my SPECIALITY, sir. :)!
I use Shreddage II in all of my projects. There's a few ways to double track. It mainly depends on what you want to do. I do metal, so in some of my newer tracks I have two differing sequences playing in each channel in some parts of the songs. So I either:
1. Have one audio channel for the two audio outputs (panned 100% L/R in Kontakt). Each instrument then has it's own separate midi stream.
or
2. Do the same thing for the midi, but this time one separate audio output for each audio channel, panned 100% L/R in Sonar's console. This is great when I want to process a slightly different tone for each channel.
Or if you just want to do something basic, both Kontakt instruments set to the same audio and midi output. Just be sure to pan them however you want (usually 100% L/R for double tracked) in Kontakt.