If you do not want to create separate master cards, you can add another financial account to the supplier card with the financial account type -> "Customer".
Thus, by adding two financial accounts to the same master card, it is reflected that the person/company is both a supplier and a customer.
Financial Account Types for Supplier Current Accounts
As explained above, in master cards with the text "ASP - Supplier", you can create multiple financial accounts with the following financial account types: