Nnyrb,
The key question is by online do you mean internet, or Local area network?. The internet always has additional security concerns.
The existing solution for this use case
I am personally working on methods to assist in this. If you want multiple users at the same time I believe your only option is to run Bob which is on top of nodeJS, a folder based wiki.
If you can get a NodeJS docker package or a host that provides node you can install bob there or the vanila node install. If you only want it on your lan you could run bob.exe on a server or desktop computer.
The generic nodeJS install has some rudimentary authentication.
Single file wikis
If you only have a html/php server you can use tw-receiver but you still have to maintain only a single editor at a time, I have not tested it but is has a feature to avoid contention when saving.
Tw-receiver and standard file based wikis is where I am working and building a check in out process to allow serial editing of file based wikis.
The next version 5.1.20 has a local storage plugin that may allow multiple editors at once with the changes exported and reviewed by the owner before adding them,
Or with work we may be able to have only one editor allowed to "commit there changes to disk" at once, using a variation of the check in and check out process.
SharePoint
Tiddlywikis can be hosted on sharepoint, and you can leverage SharePoints document management for check in and out - ask if you want to know more.
Another strategy?
You could define a network of tiddlywikis where each user has update rights to their copy and there changes can be imported into a central wiki, that then replaces the the user wikis.
A better version may be possible with Bob by sharing tiddlers accross multiple wikis in the same bob server.
Others may have a better idea.
Regards
Tony