Check this business object - BUS1076 - Financial Product (security class) (tr. SWO1) It is the only business object for FWBS and FWZE transactions.
I've never worked with workflow and business objects myself, but ABAP consultant i'm working with usually use them to triger some events. For example, when the financial document is saved (here comes some business object), another event is initiated.
In your case, when first user use "Save Without Posting" button, Business Object i mentioned might start initiation workflow. This workflow will tell second user to make Postings. This is my though about BUS1076. But you have to check it!