The normal mode of Dittoing someone else's Photon session (specifying -n) is to have a single cursor that can be controlled by either the local or remote user. Similarly, keystrokes from either keyboard can be entered into the application with keyboard focus, which is normally what you want for a remote training or debugging session.
But sometimes you may want the remote user to carry on working without necessarily being affected by what you're doing. You can do this by specifying the -u option on the Phindows command line. In this mode, your mouse, keyboard, and display are completely independent of the other user's console. Both of you see two cursors (your cursor is solid, while the other person's cursor is dimmed or ghosted), but the two cursors behave independently.
In Photon terminology, you're running in your own private input group. You're free to roam around the Photon space without affecting the other user. Your mouse clicks and keystrokes are directed to whatever application has input focus for your input group. You can therefore start up new applications anywhere in the Photon space (probably in a different Photon console to be polite). Unless the other user chooses to roam over to the part of the desktop you're working in, he or she is otherwise unaffected by your presence there.
There's no built-in limit to how many concurrent users can share the same workspace in this manner, although it's doubtful that you'd practically want to have more than two at any one time. It's far more likely that if multiple users are connected to a single QNX machine, they each run in their own private Photon workspace (the default behavior of Phindows) and communicate with each other using the built-in Photon connectivity facilities, such as Jump Gates and msgpad (QNX 4 only), and phditto. A single QNX machine can easily support dozens of such Phindows clients, provided it's fast enough and has enough RAM.