In each user's record there is a Backup ID field, just like there is a Supervisor ID field. This Backup ID field will let you set a backup approver. How it works is you set that field to the username of the user you want to back up that person.
For example, if you have three people: User A, Manager A, and Manager B. Manager A approves the timesheets for User A. Manager B should be the backup approver for this plan. In the Backup ID field in Manager A's record you would put Manager B's username. See the grid below for this in a different way.