Hi,
By making a release procedure with two release levels, you can make a release strategy for approving a document (PR/PO) by two persons. Please refer below links:
http://wiki.scn.sap.com/wiki/display/ERPLO/Release+procedure+for+Purchase+Requisition+in+MM
Release Procedure (MM-PUR-GF) (SAP Library - Purchasing (MM-PUR))