Can you have two file cabinets, one for POs and one for Packing Lists and then link them on PO number? Is this a one-to-many, as in, a packing list can relate to one-to-many POs, but a PO will have one and only one packing list? If so, then you can make a linkage such that when a packing list is being viewed, there is a link to display related POs (of which there may one or more).
This may require indexing all the PO documents with some sort of packing list ID, or adding a Keyword type field to the packing list to house multiple POs (I tihnk a linkage can be set up with a Keyword field, but I am not entirely sure -- we played with it a while back but I can't recall how well it worked, or didn't work).
We go in both directions when it coles to payables and checks. We have a file cabinet for each, and if you are viewing an AP invoice, you can link to the check that paid it, and if you are viewing a check you can link to the AP invoices it pays (which is one-to-many). To get this working, we have a check number and check date field on the AP invoice record that we populate after the fact via a behind-the-scenes automated task which gathers data to add to the AP file cabinet and then uses the Platform Service API to populate the check number and date so linkages then work.