You would have to do two assumptions of liability - one for each number being transferred to a different account.  The relinquishing party would call in to authorize it then the assuming party would call in or go into a store to accept and have the number transferred.  Keep in mind there could be safety deposits required on either side since a new person will be legally/financially responsible for the line.  Also there can not be a past due on the accounts to process the AOLs.