Hi David,
I might not be of much help as our system is configured to accept only one PO number per PD document.  

The first thing I would try is to open PD document 91626 and type TEKKI in the SAP command field.  Are both PO numbers in table /cockpit/thdr?  If so, what are the field names?

Next check the mapping of your multiple PO invoice profile fields to SAP (I am presuming that in order to get two POs for one PD document, the invoice profile allows you to Verify more than one PO number??)  Transaction /cockpit/c1 holds the mapping.  Where is the second PO going?

Ask your local Readsoft support whether there is meant to be only a 1:1 relationship between PD document and PO?  Also check with Readsoft whether there is a user exit for multiple POs per single PD document?

You could consider using /cockpit/c1 to drop the second PO into another unused header field.  For example, Invoicing Pty (/COCKPIT/THDR-DIFF_INV).  Then transaction /cockpit/c19 can be used to create another selection criteria based on Invoicing Pty. 
Quick and dirty but at least you would be able to grab the second PO if necessary.  I imagine that the majority of your invoices would be a 1:1 relationship anyway.

In short, you need to ascertain where the second PO number is getting stored before you can ascertain how to extract it.

Sorry I can’t add much more.  We are using a 1:1 relationship here.  Hopefully someone else with multiple POs per PD document can weigh into the discussion.