Adding an AWS user to multiple tenants is possible only if the email address has not been incorrectly added to a tenant as a 'regular' user.
To identify if this has occurred make a global user search for the user and inspect the returned record:
If the AWS user is associated with a customer which is not Amazon AWS, they have been added as a 'regular' user to the customer which is listed.
To enable the AWS user to be invited to multiple tenants they will need to be removed as a 'regular' user and re-invited using the AWS invite option.