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.