You can change the owner of a ticket by batch in the console. Console — Zammad documentation
First you need to identify the user id’s from both accounts.
Identify all tickets which belongs to the account you want to remove and store the id’s.
Update the identified tickets and set the new customer id.
After the change the old customer account should not own any tickets and can be deleted.
Another useful option might be using the organization feature. Create a Organization and assign both accounts to the organization. He can see all his tickets from both accounts and work with them.