In a first stage I think only having another data for billing is what have to be done. In our current soft we have lots of clients with multiple hosting who need invoices to be generated with their clients data. Our client admins these accounts and are they who pass along the invoice to their client.
This billing client has nothing to do directly with our hosting, it talks everything to our direct client, so no multiple login is needed.
This would be a nice and awaited feature, i'm making the switch to whmcs and this point was really a drawback.