You can add additional users to Approve a timesheet by going to:
Setup ->Timesheets & Hours ->Timesheet Profiles and edit the timesheet profile.
Under Approvers drop-down menu -> select Specific People -> add the user(s) name(s) that you want to be the approver for this timesheet profile and Save Changes.
The user(s) you want to select as Approvers will need to be set as Timesheet approvers before they can be selected as an approver.
When more than one user is identified as a timesheet approver they will all receive notification to review and approve the timesheet. Only one user is required to approve the timesheet in order for it to be approved.
Up to 7 users can be assigned as an approver on a timesheet profile.
When a timesheet has been approved, the other approvers will see 'Timesheet has been approved.