If you want to co-author an Office document, the following information can save you some headaches.
The people who want to co-author must open the same file/document, stored in the same location (that supports co-authoring) and it must be opened using the same type of client.
Supported Servers:
• SharePoint 2010
• SkyDrive (partial)
Supported Office Web Apps:
• Excel (no desktop support)
• OneNote
Supported Office Desktop Applications (no web app support):
• Word
• PPT
The file cannot be opened/checked-out by a client that does not support co-authoring. An example of this would be if somebody has a Word document opened for editing in the Word Web App, the document cannot be co-authored. The document will have to be checked back in by the Word Web App then Word Desktop Applications can open and co-author the document.