Multiple client instances is easy enough.
The game does a
Code:
FindWindow("TwelveSky2"..)
to see if it's already loaded (instead of a typical unique "mutex").
So either:
A) With a injected/loaded DLL hook/replace "FindWindowA" to always return NULL.
B) Use CE or something to find the right "TwelveSky2" string and change it to something else.
C) Or just change the window name of each client after you load them. There are little windows utilities you can download to do this.