What does one do with the Eruv Tavshilin food?
The bread and food used for the Eruv Tavshilin must be put away in a secure area in order so it will not get eaten or destroyed prior to the completion of the Shabbos preparations on Friday. From the letter of the law, once the Shabbos preparations have been completed, one may eat the foods designated for the Eruv Tavshilin. Nevertheless if the bread is whole, and can hence be used for Lechem Mishneh, it is a Mitzvah Min Hamuvchar to delay eating the bread until one of the Shabbos meals. Some have the custom to use the bread as Lechem Mishneh for the first and second Shabbos meal and then eat it only by the third Shabbos meal.
If the food was eaten or lost: If the cooked food was partially eaten or lost prior to completing the Shabbos preparations then it is forbidden to cook or do any more preparation on behalf of Shabbos unless a Kezayis worth of the cooked food remains. If however only the bread was eaten or lost then it remains permitted to cook and prepare on Friday for Shabbos.
 The reason: As every item which had one Mitzvah done with it should have an additional Mitzvah performed with it as well. [ibid]
 527/25 “Some are accustomed to use it for Lechem Mishneh for the first and second meal and then eat it by the third meal”; See Likkutei Sichos 16/183
The reason: As it is proper to do many Mitzvos with an item that was already used for a Mitzvah. [ibid]