Upon speaking the command word to the other mirror it too fogs and fades, eventually revealing a top-down view of the entrance through which the party entered the pyramid.

Returning to the Octagonal Mirror Room and speaking the command word, it turns out these eight mirrors are also viewing mirrors, though they simply look out towards the endless desert. Judging by the position of the fiery sun hanging in the sky, they let you view in the direction you're facing as you stand before the mirror. None of them open to provide a new passage way.