Duplicate tax registration numbers only for different parties (customer/supplier)

Summary

Customer requirement to allow duplicate tax registration numbers only for different parties (customer/supplier)

Content

Customer has the following requirement:

1. When party X is a supplier, as well as a customer, and these are not linked, to allow the customer and supplier to have the same  tax registration numbers.

2. When party X is a supplier and party Y is a supplier and these have the same registration number, to show an error message.

Customer is using supplier portal to register suppliers. These suppliers now error out when there's a customer which has the same VAT#

I have tried to configure this on the VAT regime by selecting 'suppliers' from the LOV to only have the validation work for suppliers, however this did not show any result. Any ideas on how to configure this?