QuickBooks Online Accountant (QBOA) is really a portal to access client QBO companies. QBOA also comes with one free company (QBO plus level) that the accountant can use for their own business. The Accountant Tools menu (the briefcase icon) displays few specialty tools not found in QBO.
So, you do not need anther QuickBooks software to use QBOA. The client will invite you as an Accountant from their QBO. You accept the invitation to add to portal to access client file.