U need 2 Chars with spellforce and in a server with stack subs,(i didnt try it in retail subclass server)
1-start give spell force with one char
2- the other char need to switch
3-before the other chars end to cast the spellforce u need to switch with the subclass u are ussing before switching
Done, u have more atack or Matck
Njoy
Cya
PD:tryed it on L2 Venezuela