Yes, it is an interesting possibility. Tobii has in the past prototyped different ways of synchronizing data from multiple trackers, and it could become interesting in the future to implement such a feature for a public API.
The reason why we have focused on eye trackers placed on the bottom of a monitor is that that gives the best view of the eyes. From the top of a monitor the eyes are more likely to be be obscured by eyelashes, eyelids, eyebrows, bangs, and the like. But, an eye tracker placed on the top of a monitor could of course be a good complement when the user’s eyes are directed towards the top area of the screen.