DarkGDK itself isnt thread-safe.... which means that if you have 2 threads that are doing things with GDK memory addresses, there is a very high chance that it screw one or the other pieces of data up and crash on you...
That said, I think it would be safe to have all your GDK code and any data GDK can access in one thread, and do something completely unrelated in another, iterate through a very large array applying an algorithm to stored values, vertex calculations, data compression etc, while GDK keeps rendering or doing whatever it is doing in the other thread, the trick would be to keep them seperate and ensure that one thread isnt able to access any of the data in the other, until both have completed their operations and are locked.
Lastly, in my opinion, doing the above with a non-thread safe library, that is one which was not designed to run on multiple threads, is unsafe. plain and simple. You may get away with it and avoid crashing you app for now, but its not safe coding practice, and will eventually bite u in the butt with multiple hour long debug sessions lol(speaking from experience)
If it ain't broke.... DONT FIX IT !!!