The *normal* course for mail list splits is to duplicate the entire list,
and allow each individual the opportunity of unsubscribing to one, either
or both.  Since the subscription (and unsubscription) is automated...

This avoids all of the issues of which list you end up on.  Just send a
message on each list, after the split, informing those people on the *new*
list on how to unsubscribe.

malcolm