"Coauthoring" or concurrent editing does indeed now appear to be possible in the Office 365 web version of Excel:

http://www.pcworld.com/article/2033437/collaboration-in-microsoft-office-painful-but-not-impossible.html

Here's a quote:

"You cannot coedit in Excel while one person is working in the desktop Excel. If you try, you'll get a message saying the spreadsheet is locked. But you can coauthor in the Excel Web App, which is a lot more limited in functionality. Changes appear automatically only after an author leaves a cell, and since they are not highlighted it isn’t always easy to tell what a coauthor has changed. But the app does indicate when others are actively editing."

For a while back I got this question more than once, as collaboration is a buzzword these days.  My advice to these clients after working with my Laserfiche team was to "collaborate away" in the Microsoft cloud, and then store the final results inside of LF.  Since the local Windows Excel doesn't yet support collaboration anyway, this answer should be acceptable...