Q. Is there a mitzva to eat two seudos on Purim day, the first one before chatzos?

A. Mishna Berura (695: 8) writes that the reason the seuda is not customarily done in the morning is because people are busy sending shalach monos. The activity often prolongs close to mincha time, then you are not allowed to eat unless you daven first. However, Eliahu Rabba and Pri Megodim (E.A. 5) quoting Sheloh maintain that the seuda should be done immediately in the morning after megila.
Kaf Hachaim (695: 23) cites Siddur Harashash that also asserts it should be done after shacharis. Likutei Mahariach and Kisei Rachamim mention that this was also the minhag of the G’ra.
Yafe Lalev maintains that the morning seuda should be dairy to symbolize Kabbolas Hatorah that was reiterated on Purim.
However, Mishna Berura (ibid.: 15) opines that even after having eaten the morning seuda, the second seuda consumed in the afternoon is the one called properly the Purim Seuda.
Piskei Teshuvos (ibid. n. 25) quotes many Poskim that are of the same opinion and mention, based on Cabbalah and Chassidus principles, that the seuda should continue into the night of Shushan Purim.
Horav Shlomo Miller’s Shlit’a opinion is similar and adds, that after all we eat daily breakfast, why should Purim be any less.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a