Accessing Portal in Layout locks other users out of the same Portal

I have an invoice solution. Invoice table is parent to Invoice Data table. Invoice Data table is Child to Product table. Product table is joined to Invoice Table by Cartesian relationship so that I can search a Product portal to add products to the Invoice Data Portal of the invoice. Works like a charm. I need to have multiple users accessing different Invoices at the same time AND being able to access the product portal at the same time to add products to the invoice data portal. When two users try to access the product portal at the same time Filemaker locks the record until the first user is finished. Through forum searches I understand this to be expected behavior or Filemaker. When two people access the invoice data and edit the records it does not lock the record. I am sure that it has to do with the difference between the relations ships of Invoice to Invoice Data and Invoice to Product. (I am sure I do not understand why.) What is my work around? The solution has to be able to have multiple users access the products portal at the same time. Do I change the Product to Invoice relationship? or Do I send the user to the Product table in list view instead? The second option is not very user friendly as they will have to leave the invoice to another layout and back. Scripting that is easy but not a very fluid user experience. Any thoughts or workarounds?