Dunno about forcing character creation GUI, but you could also just give the second set of characters the appropriate amount of talent, ability, and attribute points and some basic gear upon character creation completed (or maybe just on initialization). Then plop down that mirror from the End of Time right at the start (and the relevant scripts) there so those players can enter it to change their appearance. The only hard part is giving starting skills. Maybe just putting a trader right at the start that sells starting skills and give the second players some extra gold. Balance for the initial fights is borked anyway, so that's not really an issue.
A bit inelegant, but a possibility and less work than some huge dialog tree.
