That translates to - requirement is to have two level approval workflow of ROS. By SAP Standard ROS does not have multi level approval workflow.
When we have faced a similar situation in another client (701) - we created a new "Approval Field (Check Box)" which is only visible to the first level approver (with a particular custom role assigned). After the supplier submits the registration details, a email notification (customised development) is sent to the first level approver. First Approver - will tick the box and SAVE it from the supplier cockpit.
Second Level Approver (Copy of Supplier Administrator role, and is assigned only to the second level approver) will Click Approve/Accept (which is a standard button) - at the time of saving - there are following custom check
>>to ensure the First Level Approvers Check box is ticket.
>>user clicking on approve having the custom second level approval role.
But please be reminded this is all enhancement (we never did any modification) using enhancement points and really a trouble some development. But this is one possible way you can implement.