To find live editing before 2000, replace “word document” with “Text document” and have a look at SubEthaEdit. I used to use it around 2000. It inspired Gobby and Rudel.

And if you want to go further back: Just login over SSH and connect to the same emacs server using multiple buffers.

But one difference here is: The web app devs had more resources and made their applications more polished.