Overview: Co-authoring has been around awhile and I haven't paiudd it any attention in a long time. The idea is multiple authors can edit the same document at the same time.
- Word and PowerPoint use to be the only supported files for co-authoring. SharEPoint 2013 supports co-authoring in word 2013, PowerPoint, OneNote and visio 2013 as of 20April 2016.
- Excel does not provide co-authoring unless you use Office Web Apps or excel services. More info
WIP -Versioning issues with Word....
Article describes nicely what applications work with co-authoring https://technet.microsoft.com/en-us/library/ff718249.aspx