Hi everyone! Thank you so much for all of your feedback and interest in this feature over the years. I’m very happy to say that it is now possible (in beta!) for Service Hub Enterprise accounts to host multiple knowledge bases with distinct domains, branding, and content -- all within one HubSpot account. 
If you filled out the form I linked here previously and you have Service Hub Enterprise on the Hub ID you sent in, I’ve already added you to the beta. Please let us know if you have any feedback as you test the feature out!
You can read more about how to set up multiple KBs in this article.