It seems pretty ridiculous that there’s a TogetherJS plugin for WordPress, and MS Sharepoint boasts collaborative editing, but MW doesn’t have this feature. You can do real-time collaborative edits in Google Docs.
C. Scott Ananian started the Extension:TogetherJS project back around 2014, but it hasn’t gained any traction. The code is abandoned, but seems like it may actually still work. I’m discussing the status and might make a fork if needed.
TogetherJS is a project of the Mozilla Foundation
Here’s a thread about using TogetherJS