As R. Herschel Schachter observes (Eretz HaTzvi, 4) this second explanation indicates that the nature of the obligation Pesach night is to have a meal of matzah, rather than there being an obligation to eat maztah, and a separate Yom Tov obligation to have a meal. This appears to be the position of the Rambam as well. He states (Hil. Chametz U’Matzah 8:6) that two matzot are used, and one is broken, in fulfillment of the concept of “lechem oni”. Tosafot, however (Pesachim 116a, s.v. mah darko), differ, and understand that there is a requirement of lechem mishneh in addition to the broken matzah; the Gra (biur to O.C, 473:11) observes this view is shared by the Rosh and the Mordechai.
Their dispute is apparently around the above issue.According to the Rambam, the obligation Pesach night is to have a yom tov meal of matzah; thus, there is no need for lechem mishneh of two full maztot, as lechem oni teaches that one of the matzot should be broken. According to the other rishonim, however, the obligation of matzah is independent of the obligation of the Yom Tov meal, hence the need for three matzot, two for lechem mishneh and a third for lechem oni.
Further, the Rosh and the Mordechai are thus consistent with their views. Those rishonim hold that on Pesah night, after the berakhot, two k’zaytim of matzah must be eaten, one from the broken one for the mitzvah of matzah, and one from the complete one for the Yom Tov meal. The Mishnah Berurah (O.C. 475) questions this position. According to the above, their position can be understood; they view matzah and the Yom Tov meal as two separate mitzvot, and because of the rule of “ein osin mitzvot chavilot chavilot”, it is improper to fulfill both mitzvot on the same piece of matzah.