i wonder about that. If the quality goes past a certain point i think so. But i dont think we will be getting military grade tracking here. Sampling rate will determine how small of an area they can get away with rendering at high resolution and still keep up with eye movements. If say that size is 40 percent of the screen, then tracking one eye will encompass any difference in where other eye is focusing.
If it was say 10 percent of the screen, allowing for really aggressive gpu savings then you might need to track both, but i have no reason to believe we will be getting something sophisticated enough to do that.
Moral is, the more aggressive the foveation, the better the tracking needs to be to make that work. i would be happy with something like what is currently being done with the game raw data, but which moves with eyes.