Less than 1/48th missing is still considered a whole loaf. So if you only have two challot (or other loaves of bread) for Shabbat, you might be able to use one loaf twice, as follows:

• Wash your hands,
• Say ha'motzi,
• Cut off a piece that is less than 1/48th of the loaf, and
• Eat it.

REASON You may consider the remainder of that loaf as still being a full loaf and you may re-use it for your Shabbat morning meal.
NOTE If you have pieces of bread or other mezonot, you may:

• Cut off less than 1/48th of the loaf,
• Eat the additional pieces of bread to make a total of at least 1.9 fl. oz. (56ml), and then
• Re-use the same loaf for Shabbat morning.