There are situations in which Shabbat lunch is drawn out to such a point that it extends into the time when Seuda Shelishit can be eaten. In such a situation, it is not hard to imagine that one would not have enough of an appetite to have Seuda Shelishit later on and may come to forego it altogether. As such, the Shulhan Aruch’s (O.H. 291:3) solution is that one break the lunch meal in half by reciting Birkat Hamazon, and then washing Netilat Yadayim on bread again. The first half of the meal will be considered Shabbat lunch, while one can fulfil one’s obligation for Seuda Shelishit during the second half. Nevertheless, the Rama (ibid.) adds that one should not rely on the Shulhan Aruch’s solution if one believes that one will be able to eat Seuda Shelishit after praying Minha, since there is Halachic significance to this order.
Rabbi Shlomo Ashkenazi Rappaport (Lev Shelomo, § 4) wonders why the Shulhan Aruch’s recommendation is not considered an unnecessary blessing. After all, the blessings of Netilat Yadayim, Hamotzi and Birkat Hamazon could theoretically be only recited once for the entire meal, even if it stretches several hours and there is no apparent justification to recite them twice. Nonetheless, he quotes Rabbi Moshe Galante who, while discussing the concept of reciting one hundred blessings a day, rules that one may come in and out of a room repeatedly, whie each time reciting a blessing on a piece of food, and says that this is not considered unnecessary. Rabbi Itzhak Tayeb (Erech HaShulhan) concurs. The HIDA (Birke Yosef, § 46), on the other hand, does not think the two situations are analogous. When it comes to the example of coming in and out of a room in order to recite the same blessing repeatedly, there is room to argue that it could be a violation of an unnecessary blessing, since there are other ways one may attain one hundred blessings a day. In the case of splitting the meal, however, repeating the blessings are, to the contrary, quite necessary. If one were to eat a large meal and later had to eat Seuda Shelishit while still satiated, one may be forced to engage in Achila Gasa (lit. “over eating”) which is inappropriate, or worse yet, one may feel the need to forgo Seuda Shelishit altogether.
Summary: If one will not be able to properly fulfil Seuda Shelishit because Shabbat lunch is long, one can split the lunch meal into two.