I can't manage to produce on the real-time WYSIWYG editor developed in XWiki platform. I didn't try with two actual live users, but I tried like this:

• logged in from Chrome with some user
• logged in from Firefox with a different user
• edit the same page on both browsers
• on Firefox type some line of text
• on Chrome type another line of text then open the JavaScript console and execute easyTest() to start the automated typing (which should simulate another user typing at the same time)
• on Firefox type at the end of the first line and then use Backspace to delete all characters one by one

None of the characters deleted with Backspace were added back.