Supplier portal - Can parent/child suppliers be accessed with one user account?

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We have suppliers that would like to use supplier portal that have Parent/Child suppliers in our system.  Is it possible to access all of the suppliers in the parent/child hierarchy through one supplier portal user account?

We've tried setting up the same contact on each supplier in the hierarchy but get an error due to the duplicate user name.  We are using email address as user name.

Regards-
Melissa