How do I open multiple Tiddlywikis in a browser, but the body content is the same tiddlywiki, and each one is edited, finally and saved?

What did you mean here?

There are a number of ways to develop workflows to assist here. They in part depend on if you are using a single file wiki, a node server or Bob server.

To cut a long story short with single file wikis try to only ever open it in one tab or window at a time. You can achieve much of what you seem to want by selectively opening tiddlers you want to read along side each other using a different layout A new Flexbox Layout for TiddlyWiki or using the tiddler button Open in New Window.

See also;

  • Muuri
  • MCL
  • Flexbox
  • Stories Plugin
  • Streams (oultliner)
  • Mentat ?

Open in new window uses a default template which can be altered to allow other “views” within the new window, such as edit a tiddler, or a separate story.