Recently published Co-authoring administration (SharePoint Server 2010) TechNet article provides guidance for administrators to configure co-authoring for their organization. Many of these are using Windows PowerShell cmdlets.
note: File Synchronization via SOAP over HTTP Protocol makes co-authoring work. In case you want to know more about it, download and read the PDF from [MS-FSSHTTP]: File Synchronization via SOAP over HTTP Protocol Specification page.
I would like to clarify if it necessary to start the Excel Service Application to use the CO-Authoring option or only Excel Web Apps is required?