Mark,
Perhaps Jed can answer that. It could be great,
I think at the moment Bob (the single Host) is dealing with the possible contention between multiple users on a per tiddler basis. There may need to be an extension to allow two hosts to share the one file area, but its a bloody good question. However on the current model if you can get a node server accessible on the Internet both can happily visit the one tiddlywiki and both work on it, with only changing tiddlers where contention is managed.
I recently hosted NodeJS on the internet, and plan to make this bob however I am cautious because it can be a very high security risk. But Jeremy is working on a Login security component.
With Vanilla Node or even TiddlyServer it can auto-save every change quite quickly, I think it would be quite easy to, check it is available for checkout, save a tiddler indicating I have the wiki checked out for editing, so others can view but not save, until I check it back in, then they can check it out.
I believe this is possible with the current technology. Bob can help in this scenario to avoid the need to save and reload the wiki.
Another method that may work in some use cases is to force people to "login" to the wiki, basically providing their user name, them allow them to create and edit tiddlers created by them but not others (unless they check it out as above). Ie check it out to edit system tiddlers, or others tiddlers, otherwise use it "not checked out" knowing users will not step on each others toes.
Just some recent thoughts of mine
Tony
Basically this would allow multiple user edits over time if not at the same time, or what I would call "serial editing". In many business cases this is often more than enough.
Regards
Tony