If my understanding is correct we use two Challot on Shabbat to represent the two portions of mana the Jews received in the desert of Friday to prevent them from having to collect on Shabbat. We use the salt to represent the 'preservative' Hashem put on the extra portion of mana to keep it until Shabbat (every other day the extra the Jews had would spoil). (I know there are other reasons for the salt, but I have never heard of another reason for the Challot).
If this is so:
- Why do we use two challot on Shabbat day?
- Why do we use salt on Shabbat day?
Neither of these represent what happened in the desert.