The signoff authority is the feature that you can use for this purpose . From the official documentation:

To transfer signoff authority for yourself or others :
1. Choose Settings > Transfer Authority in the main menu bar. The Transfer Authority dialog box
appears, which lists the transfer authority definitions you have the ability to modify or delete.
2. Click the New button. The New Transfer Authority dialog box appears.
3. Select yourself or another user to transfer authority from, then select the user who is receiving
signoff authority.
Note Users who are receiving the authority must have the appropriate privileges to
sign off the routable objects transferred to them. If you have questions about
privileges, see your Agile administrator.
4. In the Select Criteria list, click to select the type of information to transfer.
To view the filter conditions for the selected criteria, click the View button in the Criteria pane.
The criteria is displayed, much like a search definition, in a read-only dialog box. You can verify
whether the selected transfer criteria will find the routable objects you want to transfer. Click
Cancel to close the dialog box.
Note The list of criteria is quite extensive. If you have questions about which criteria
would best suit your needs, contact your Agile administrator, who manages all
global criteria. Alternatively, consider defining a personal criteria that you can
then use for transferring authority. See Defining Personal Criteria for Authority
Transfers (on page 229) for information about creating personal criteria.
5. Specify a start date and an end date.
6. Under Affected Changes, specify whether you want all routable objects in your Inbox to be
transferred, or only routable objects that arrive in your Inbox within the date range you
specified.
7. Click OK to complete the transfer.
The new transfer authority definition appears in the Transfer Authority dialog box.
8. Click Close

Hi Mooseman, I’d like to suggest an alternative to the very well described solution offered by Antonio above.
Create a new personal usergroup containing two users, the supervisor and yourself. Be sure to add all necessary roles to this usergroup.
On the workflow approval step that sends the change to the supervisor, change his user, in the approval field, to the new usergroup name. Be sure to configure the step to allow only one signoff by group.
Both will receive the approval notification and in case you are out-of-office, your supervisor could approval the change.

My Best
Carlos