Registering the same contact with multiple suppliers?

It is very common for large companies to have subsidiaries operated by the same employees who use a common email domain.

Example:

Oracle Corporation and Oracle Systems Corporation are two separate entities whose employees all use "oracle.com" as their email domain, and some employees perform actions under both companies. This is just an illustration. I have real examples with companies like GE and Air Products.

Is there a way in EBS that I can create the same person as a contact under both suppliers and have them access iSupplier Portal and Sourcing? My attempts are resulting in "Error: A user account with this user name already exists.".