Cannot share an Excel 2010 file on SharePoint Foundation 2013

I have a .xlsx file on a network drive and using the Share Workbook feature without any problem.

But when I try to use this feature with a file stored on SharePoint, it doesn't work. When [User A] opens the document, [User B] can only open it with Read-Only.

Answer

I'd consider that a blessing. Sharing Excel workbooks only leads to trouble, because it's a question of when not if the shared file becomes corrupt and unusable.

If you require simultaneous multi-user write access to a data set, Excel is never the best solution. Consider a database like SQL or Access. You have SharePoint, so you already have SQL. Make the best of it and create an Excel user interface to read/write the data into a SQL database.

Simultaneous file editing is currently supported for Word and PowerPoint and is on the cards for Excel Web Apps, but not yet a reality. Watch Office 2013 and the respective web apps. While still on 2010, stick to the tried and trusted. Avoid shared workbooks.