Hi @spacey, the larger the presentation screen, the further away this will need to be from the eye tracker to allow the full range (angle box) of gaze data to be captured which of course will necessarily impact the accuracy of the data.
There will be a dead zone of sorts beyond the trackable range, but this will depend quite specifically on the angles, distances, etc of your overall setup.
In terms of the Tobii Analysis Software capabilities to overlay multiple user data, indeed that is a common feature and one that is well supported so minimal effort is required on the users part to achieve this.
However, given the specific and customised nature of your setup and the need to establish ahead of time the right hardware and environment for your needs, please contact Tobii Pro Support directly @ https://www.tobiipro.com/help/ and they should be able to guide you from experimental design up to the final sale.
I have already informed them of your case, so you should receive a reply promptly. Thanks for your understanding.