I don't see any reason why you can't have them together on the same machine (it's just software after all). Plus, I've run GS3 together with versions of Kompakt using DFD extentions without a problem, so I can't imagine that KS3 would be functioning any differently. However, you are right in that it's not particularly a great idea to run them simultaneously, but as a matter of opinion, if your PC up to the challenge you are welcome to use the "Big Hammer" approach whenever you wish. It just depends on what you need to achieve out of the project.
Of course, you can always use GS3/K3 to get the tracks you want, bounce the tracks to Audio, then disconnect GS3/K3 to free up resources for the other sampler. That way you get to have your cake & eat it too!
I haven't tried GS3 like that though. Standalone for both apps?
I use Scope DSP cards w/ their ASIO 2, and GSIF drivers. Personally I prefer the editing in GS3 for the creation of GVI performances, then use the ASIO 2 drivers so I can use the best of K2, and Gigastudio libraies.
There are quality developers for both platforms, I'd hate to miss out on anything by staying with just one or the other.
However, it will be interesting to have Gigastudio 4 hosting K2, or K3. Whatever works, I will use. If GS4 is as buggy as GS3, no problemos. I will stay w/ 32bit XP, and use the 4GB's of RAM w/ the 3GB Switch.
I like options. Giga and Kontackt are rivals, but both are friends of mine !!