In Vayikra 19:27 the verse says לא תקפו פאת ראשכם ולא תשחית את פאת זקנך and it means both Peyos on the head yet in Vayikra 23:22 the verse says ובקצרכם את קציר ארצכם לא תכלה פאת שדך בקצרך ולקט קצירך לא תלקט לעני ולגר תעזב אתם אני יהוה אלהיכם and it means only one edge of the field. Why does Pe'as by the hair of the head mean both sides yet by a field it means only one edge?
The Sefer M'rafsin Igra V'yakel pg.199 asks this question. Rav Moshe Brim and Rav Yair Yedidya Pacha answer and explain the reasoning of the mitzvah. They explain the issur of not leaving over a corner comes from the mitzvah of leaving over for the poor. Therefore if one would leave over four little corners or one big corner there would be no difference,as long as something was left over. The issur of rounding the corners come from the issur of chukas hagoyim therefore one gets the issur if he rounds one or both of them.(see inside the sefer for another answer by Rav Moshe Landau)