This mishnah deals with the halachah that a person may not eat the Pesach offering with two separate groups. The members of a group are required to remain with their own group and their own Pesach offering, and they do not join another group. One group may divide into two groups, but once they have divided, no member of the one group may join the other.
Two groups eating in one house:
The mishnah rules that these turn their faces in one direction and eat, and those turn their faces in the other direction and eat. They are not required to face one another so as to appear as a single group, since two groups are permitted to eat in one house, and therefore they may make it clear that they are two separate groups.
The urn, the vessel of hot water, stands in the middle, and both groups make use of it. Even though this too makes the distinction between them apparent, there is no concern in the matter, since two groups in one house are permitted.
The attendant who serves both groups:
An attendant who serves both groups and eats with one of them, when he rises to pour and dilute the wine for the second group, is subject to two rules:
"Kofetz es piv" - he closes his mouth: if he has food in his mouth, he may not swallow it once he leaves the area of his own group, lest he be found eating in two places.
"Machzir es panav" - he turns his face back: when he goes to the second group he must turn his face back toward the group with which he is eating, to show that he is still counted among them, so that he not be suspected of eating with another group.
He must keep his face turned back the entire time he is serving the second group, until he returns to his own group, and there he completes his eating.
The bride:
A bride, a young woman who is embarrassed to eat in front of others, may turn her face away from those reclining with her and eat, even though it appears as if she is eating in a different place from everyone else. This rule applies even if she first ate part of the meal without turning her face away, and in the middle of the meal she became embarrassed and wished to turn her face away - this is permitted, and it is not considered as eating in two places.
In summary: a person does not eat the Pesach offering with two groups, but two groups may eat in one house with their faces in different directions and the urn between them; the attendant who serves both of them closes his mouth and turns his face back until he returns to his own group; and the bride turns her face away and eats because of her embarrassment.