Show an entry is being worked on Is there anything that will prevent an entry from being worked on by 2 editors at the same time?
Nope, not currently... though there is a feature request for it:


  http://feedback.craftcms.com/forums/285221-feature-requests/suggestions/9009169-add-an-alert-or-warning-when-multiple-authors-are


I guess it's possible for it to be done in a plugin, although I'm struggling to picture exactly how. At this time, no such plugin exists.