Q. Is document co-authoring enabled by default?
A. Yes, in SharePoint 2010 and 2013. Co-authorship is the default behavior of document libraries. Multiple users can access a single document at the same time with Office 2010 from a SharePoint library. A good general practice is to turn Track Changes on from the Review tab. For the document owner, it makes editing the finished product much easier.
You can turn this feature off. To do that, go to Library Settings and choose Versioning Settings under General Settings. Scroll down until you see Require Check Out and click the Yes radio button.