Lechem Mishneh: Otzar ha-Kavod
The top and the bottom loaves are said to represent Binah and Malchut, respectively. One is to consume only the bottom loaf, thereby indicating that Binah and the Uppermost rungs are beyond direct experience.
Rabbi Ashi said: I saw that Rabbi Kahana held two loaves but broke bread over one. So it is fitting because the one left unbroken is the “Eye” – Ayin – of which it is said, “No man has seen the Eye, O God, but You.” (rereading Isaiah 64:3) For even the greatest of all the prophets did not merit this, as Scripture says, No one can see My face (pannai – the innermost aspect) for no man may see Me and live.”