Commentary for Kiddushin 134:6

If you say that the mishnah accords with R. Yehudah who said that a convert may not marry a mamzeret, then the mishnah works well. The words “in any case” come to include an Israelite who marries a halalah (the child follows the father and is not a halal) and the case of the second generation Egyptian who marries a first generation Egyptian. The words “this is the case” exclude the cases of Ravin and R. Dimi from above.

If the two non-Jews convert, we follow the inferior status. The statuses compared here are Egyptian—one can marry an Israelite after the third generation, and Ammonite, one can never marry into Israel (again, these statuses no longer exist). But the prohibition of Ammonite is only against Ammonite men, not women. Therefore, it must refer to an Ammonite man who marries an Egyptian woman and not an Egyptian man who marries an Ammonite woman. If the child is male, then he is considered an Ammonite who may never marry an Israelite. If the child is female, the Ammonite prohibition would be meaningless. Therefore the child is considered a second generation Egyptian. This girl’s child will be able to able to marry an Israelite.