I use Sonar 5 and just installed K2 last night. I'm using it as a VST in Sonar. I read that standalone is good if you are editing samples or using K2 for live performance. Standalone allows access to 64 instruments versus 16 in vst mode but you have to have your sequencer connected to it via a virtual cable. I didn't explore that yet.
Also, each of the 16 instrument slots available in VST mode has its own 128 bank whcih can be accessed via midi control changes. Page '58' of the manual is vague. You have to go to 'Open new instrument bank' to use this feature. Place your instruments in the slots of the 128 bank, and switch them via midi controal changes as an alternative to key switching!
I don't know how to use it as a standalone in Sonar, but with EWQLSO Gold, at least, I used to use it as DXi, but someone on these forums told me that a bug with the DXi was the reason I had notes sticking sometimes if I decreased tempo "too much" (usually hardly at all). This problem so far does not exist with the VST plugin.
You can run multiple instances of standalone, too. Make copies of the K2 icon and open each. I think standalone is a better option - especially CPU wise, but requires some extra routing to make it work.