Lechem Mishneh: Zohar: Upper & Lower Wisdom
The Mystery of the two loaves: One should break over two because there is to be a ‘double portion” on Shabbat. This is well and good because the double portion of bread alludes to supernal wisdom called “Bread” – Chochmah/Binah – as it is written, “Asher’s bread shall be rich.” (Genesis 49:20) It also hints to Elokim, “For bread is Elokim’s.” (Leviticus 21:22)
The first is called “Lechem Ashir, (as in Asher), enriched bread, while the second is called “Lechem Oni,” the bread of poverty.
Since we are enjoined to bring together the Upper and Lower Wisdom, we break over two loaves.
One should actually break the top loaf because the bread, the blessings, of Shabbat issue from the upper one. (Zohar 3:98a)