You could try the following:
- Go to Joplin's settings, to Keyboard Shortcuts.
- Search for the command "Indent more".
- Edit the keyboard shortcut to
Tab(by pressing on the Tab key).
Yeah... This is a frequently requested (and technically challenging) feature (example), and even a proposed GSoC project (didn't happen this year, as far as I know). For now, you can define an external text editor that will work in tandem with Joplin, and use it to open your notes in multiple windows.
More info on the aforementioned plugin.