How can we achieve Cumulative Requisition Approval within standard workflow?

What are the options available to get a cumulative requisition approval within standard workflows?

We are using standard workflows for requisition approval.

Suppose there is a SOW (Scope of Work) for $50,000 for which a requisition # 1 is created in the system (iProcurement) to get the approval and create a PO for the same.

Later when there is an increase in SOW, we create requisition # 2 for e.g. $30,000 and send for approval. At this time the approval should go for cumulative approval of $80,000 instead of just $30,000.

How can we achieve this?

Currently we are manually adding approvers taking into consideration cumulative amount ($80,000) for requisition approval instead of just $30,000. How can we eliminate this manual process?