PO numbering for Multiorg in R12

We are upgrading to R12 from 11i.  One of the biggest benefits when it comes to invoice entry is the fact that the invoice entry form is across organisations.  You are presented with a list of values for PO numbering, you enter your PO number and the vendor, operating unit etc will populate from the PO entered.  The issue is when the same PO is in multiple operating units.  You are then presented with a list of values all showing the same PO numbers and you need to choose the correct one.  We would ideally like to segregate our PO