I would love to have a multi user system (based on linux) using only one X11 session with multiple screens and pairs of mouse and keyboard. So two (or more) people can work with the same computer sharing not only the same hardware but also the same "screen" (which would be split into two physical screens of course, but you could move a window to your partner for example...). Sharing the windows should not only make it more convenient to "show" your partner what you have done - if user A started to work on something using a complex application (assert that it wouldn't be convenient to save the files and open them in the other session) moving the window of the application to user B should be as simple as moving a window within your own screen. That's why I call it a "seamless" multi user session.
I read about X11 "multi seat" in this article which doesn't have the features that I want. It uses a session for each user rather than one single session.
I found XI2 aka Xinput2 which provides a multi-pointer support. This allows having two separate mouse pointers controlled by two mice. I read that you can assign two keyboards to the two mice providing independant focus and text input. But I wonder if the clipboards (both "real" and "middle mouse button" clipboards) are treated separately too... I found only few information on XI2 multi pointer feature but no "field report".
Another, completely different idea would be having two separate X11 sessions on the computer but share the windows using X11-forward between the two sessions. BUT: As far as I know, you can not share a X11-forwarded window so that user A runs an application and while it runs, send the window to user B. As I know, only user B can run an application on the hardware of user A and display the window on it's own X11 session. That's again not what I want... Or am I wrong and it is possible to forward a window via X11-forwarding AFTER the application has been started?
Any other ideas to get this done?