Why is it traditional to use two challot on Shabbat?

Tradition holds that two whole challot should be used on Shabbat as a remembrance of the double portion of manna that fell in the desert so that no Jew should have to gather food on Shabbat (Exodus 16:22-32). Another interpretation is that the two challot fulfill the biblical injunction articulated in the two versions of the Ten Commandments in the Torah: "Remember the Sabbath day and keep it holy" (Exodus 20:8) and "Observe the Sabbath day to keep it holy (Deuteronomy 5:12).