Can requisitions be approved by actual approver even with delegated approver in place ?

Hello,

We have employee "A" who will be on vacation and has setup requisition and change orders approval delegation to employee "B".

The delegated employee "B" needs to act as an optional approver only in the event where employee "A" is unavailable.

We are looking for an approval group as the final rule to include both employee A and his delegate employee B where the approval would be complete with whomever actions first. 

Is this something we can achieve within BPM/Approval rules ?.

Thanks,

Chandru