How to enable automatic sign on to Oracle ireceivables if users login to other customer portals usin

Hi Support, We are evaluating rolling out ireceivables for customers who currently access their open invoices through our custom portal for RS that looks in R12 views for open invoice information and allows for payment) So if we switch from Custom portal to irecievables we will need to create the customers as self service users and they will have separate user ids in Oracle. One of the requirements that we have been given is the customers who log into custom portal for their ordering process should not have to log into separate system for accessing their open invoices and making payments.