We have the backend infrastructure mostly in place for real time collaborative editing. See:
https://wikimania2014.wikimedia.org/wiki/Submissions/Real-time_Collaborative_Editing_with_TogetherJS
But actually rolling out collaborative editing requires more than just the Visual Editor team. It will require a coordinated effort to reimagine what editing is like. Some tentative steps were discussed at:
https://wikimania2015.wikimedia.org/wiki/Submissions/Real-Time_Collaborative_Editing
We will need mechanisms to create user groups, real time chat mechanisms, mechanisms to temporarily persist collaborative sessions, perhaps even new core mechanisms for describing revisions (at the very least, revisions can have multiple authors!). We also need to think about social mechanisms and preventing harassment and vandalism of collaborative sessions. @Pginer-WMF has sketched out a vision for the UI, but we need to achieve broad consensus on the overall direction, refine the details, and figure out how to get there from here, with a unified roadmap across design, services, and editing.