I guess things might get nasty when you try to _run_ simultaneously, unless they actually have implemented a CRDT for the whole notebook state (it would still be nasty from a usability POV, but at least it would be consistent). Just writing on the notebook would work just as any other collaborative editor out there, I expect.
JupyterLab does use CRDT via yjs (https://github.com/yjs/yjs), and the JupyterGIS collaboration system is based on the service provided by JupyterLab. So multiple users can edit the file concurrently.
