After eating the maror, we make a sandwich from a kezayit of maror in a kezayit of matza and dip it in ḥaroset. Some people also shake off any ḥaroset that sticks to the maror, as with the eating of maror (MB 475:19). Others do not remove the ḥaroset from the maror in the case of korekh (Kaf Ha-ḥayim 457:32). We then say “zekher le-Mikdash ke-Hillel” (“in commemoration of the Temple, according to Hillel”), and eat the korekh while reclining (SA 475:1). As we learned, a kezayit is about a third of a machine matza.
According to Hillel the Elder, in Temple times the mitzva was fulfilled by eating matza and maror together, as it is stated: “They shall eat it with matzot and merorim” (Bamidbar 9:11). According to the other Sages, people would eat the matza and maror separately. Since no final decision was reached regarding this matter, we follow both practices.
However, even Hillel would concur that today it is impossible to fulfill one’s obligation by eating matza with maror. This is because without the Paschal sacrifice the mitzva to eat matza remains Torah-based, while eating maror is of rabbinic origin. If they are eaten together, the maror, which is rabbinic, will detract from the matza, which is from the Torah. Therefore, one must first eat a kezayit of matza. After this, one eats a kezayit of maror without matza, because the matza obligation has already been fulfilled, and if one eats matza with maror at this point, the taste of the matza, which is no longer a mitzva, will overshadow the taste of the maror, which is a rabbinic mitzva. After fulfilling both mitzvot separately, we put matza and maror together like a sandwich and eat them in commemoration of Hillel’s practice (Pesaḥim 115a and Tosafot s.v. “ella”; MB 475:16). And some say that while it is clear that today, according to Hillel, the matza must be eaten separately, nonetheless, in order to fulfill the rabbinic mitzva to eat maror, it must be eaten with matza. According to this opinion, when we eat the korekh, we are fulfilling the mitzva to eat maror (Pri Ḥadash).
In any event, according to all opinions one should be careful not to talk until he has finished eating korekh. Indeed, it is customary to refrain from talking between the blessing over the matza and that of the maror, until we have finished eating the korekh. It is only permissible to talk about matters related to fulfilling the mitzva.