For the person that's away, they can set up a Proxy ("Account Settings", "Proxy"), specify a resource and start/end effective date that the proxy resource should receive notifications/workflow actions/etc. in their absence.
Alternatively, you can change the approval workflow to send the action to a Group instead of a name resource, and that way everyone in that group will receive the Approval action item, and you can then dictate in the workflow if you want "first in" approval, or "all" to approve, etc.
Hope this helps.
Marlon.