How to group PPM invoices into single AR Invoice

Summary:

We are creating two invoices from PPM and interfacing them with Accounts Receivable. We want these two PPM invoices to create a single invoice in AR, however, as the transaction number attribute is different in the AutoInvoice interface, two invoices are created in AR.
The transaction source used is "CONTRACT INVOICES," and the autoinvoice grouping rule is "XX PROJECTS GROUPING RULE," which we have already confirmed is not causing the separation into two.
How can we get the two PPM invoices to create a single invoice in AR?

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):