Hi Rufino,
I moved your questions to a new topic.
The EyeX Engine handles multiple user calibration profiles. This means that users can create their own calibration profiles, and switch between existing profiles in the Tobii EyeX Settings Panel. Once a calibration profile has been created, the user does not have to re-calibrate.
The eye tracker can only track one pair of eyes at the time, so it cannot handle multiple users at the same time.