Can a customer-facing portal, for customers to view invoices and make payments, be connected to two separate gateways for two subsidiaries (Two businesses' invoices presented in the same portal, but payments route to a separate, respective gateway)? If this is not possible, does your customer-facing portal allow for us to design a lander with login credentials that can O-auth to log in to an SAP customer-facing portal?
This would be for SAP S/4HANA