OK - dumb question, but you would be suprised how often it catches people out.  Are you sure the notes you are playing are in the instruments range?  Pre-Bass as a lot of key switches and if you are playing them you will hear nothing.

Well, as long as your midi keyboard controller is active in Sonar, and you have the midi track set to output midi data on channel 2, and you have the instrument loaded in Kontakt set to channel 2, and you are playing midi notes that fall within that instrument's playable range of notes, there's not too much that can go wrong.  (or is there?)
 
In addition to the above, there is still the matter of the audio track routing assignment for its input, to consider.  If you have all of the above set up correctly, then I would think maybe the INPUT assignment for the audio track you have in your project for playing the output from that 2nd instrument that is loaded in Kontakt may not be set properly.
 
Check that audio track;s INPUT routing assignment, and make sure it is picking up the proper output from Kontakt's Output Section.  If you recall from my earlier instructions, Kontakt by default will send all loaded instrument output to the 1st set of audio outputs it has, and you have to add audio output channels to Kontakt's Output Section and set them up for each loaded instrument.
 
What I had explained in those instructions was that MY approach to doing the above is to use a BATCH Function in Kontakt to Clear all output assignments and Create a New Output for Each Loaded Instrument.  (Then I rename those to something more meaningful).
 
So, after checking your audio track input routing assignment, go back into that instance of Kontakt, and make sure you have your 2nd loaded instrument sending its output to the correct audio outputs in the Output Section.  There are 2 parts to checking that.  First, look at the 2nd instrument in the multi-rack and you will see where it is sending its output.  THAT has to be correct, AND also check the Output Section to make sure you have set that up properly, so that there are separate distinct outputs present for EACH loaded instrument in the multi-rack.
 
Please check the above and post back, and we will get you going.
 
Bob Bone
 

I think I've worked out the OP's problem here. He's setting the MIDI track's input to Ch.2, not the MIDI channel of the track itself. 
 
The MIDI track's input is the channel on which it receives MIDI data, i.e. from your controller. The MIDI channel of the track is the one that you set in the "C" dropdown (underneath the FX bin). Set this to "2:Kontakt" and you should be fine (provided the output of the track is set to Kontakt). 

You know, I thought about that too, but had to run out and didn't ask that question.
 
To the OP - well?  :)  (Could that be what is happening?)
 
Bob Bone

I'm going by the following quote from his last post: "HERE is where things go wrong. I set the MIDI track's input to MIDI Ch.2"

Default setups for Kontakt are flakey and need some setup.. then saved as a template in Sonar so it works next time you need it..
 
Sonar & Kontakt audio setup...
 
http://www.youtube.com/watch?v=KjWyt0DVRqM

Ok, when you've got the second library loaded (bass in my example), if you click on the virtual keyboard in Kontakt, do you hear the sound of the bass?
 
If not, I'd have a look at Kontakt's current output assignment.
Make sure the output to which the bass is routed... is routed to an audio track in Sonar.

If you have more than one instrument loaded in Kontakt you need to select it in the instrument pane to access it via the keyboard in Kontakt.. This is the bottom left pane. 

sharke
I'm going by the following quote from his last post: "HERE is where things go wrong. I set the MIDI track's input to MIDI Ch.2"

Good catch on that one, Sir.
 
To the original poster, please review this and respond.
 
Bob Bone
 

You have enough knowledgeable folks to help work this out for you... but once you understand how the routing works with KONTAKT/SONAR it really is quite simple... not to minimize your troubles to the point now where I can fly in midi tracks and kontakt audio tracks at will without really thinking about it. You too will get there.
 
Jim