PO Approval Revoke

Dear Gurus,

We have set up the PO release strategy with 03 levels.

Once the PO is fully approved ( all 03 levels released), any middle level person can revoke his approval.

Once he has done that the higher level automatically gets revoked.

( That means if the 1st level approver revokes , both 2nd and 3rd approval revoked automatically. If the 2nd level approver revokes , 3rd approval revoked automaticcaly

I need to contorl the same in such a way that, the 1st approver can revoke only after the 2nd level approval is revoked. Simerlerly the 2nd level approver can revoke only after the 2nd level approval is revoked

Please guide.

Thanks in advance.

NalindaR