How to register one email address of external customer for multiple Customer Accounts?

Summary:

There are multiple Customers accounts for an Enterprise and wants to have a common email id to get registered for Bill Management for all underlying Customer accounts

Content (please ensure you mask any confidential information):

We have created contacts on 2 different customer accounts with same email id and same name.

We would like to register this contact to get ‘Bill Management’ portal access for both customer accounts.

i.e. Once this user signs in to Bill management, then he/she should be able to view the invoices for both customer accounts.

When we tried to register this email id in ‘Manage Bill Management Registration’, registration happened only for one customer account and not for other.