Responsa for Yevamot 201:1
היו שניהם כהנים וכו': תנו רבנן הכה זה וחזר והכה זה קלל זה וחזר וקלל זה קלל שניהם בבת אחת הכה שניהם בבת אחת חייב רבי יהודה אומר בבת אחת חייב בזה אחר זה פטור
IF THE TWO [HUSBANDS] WERE PRIESTS etc. Our Rabbis taught: If he<span class="x" onmousemove="('comment',' The son concerning whom it is unknown, as in our Mishnah, which of his mother's two husbands was his father. ');"><sup>1</sup></span> struck one<span class="x" onmousemove="('comment',' Lit., 'this', one of his mother's two husbands. ');"><sup>2</sup></span> and then struck the other, or if he cursed one<span class="x" onmousemove="('comment',' Lit., 'this', one of his mother's two husbands. ');"><sup>2</sup></span> and then cursed the other, or cursed them both simultaneously or struck them both simultaneously, he is guilty.<span class="x" onmousemove="('comment',' Since one of the two is certainly his father. As to the necessary caution v. infra nn. 12 and 13. ');"><sup>3</sup></span>
Teshuvot Maharam
A. Bediabad, in this case, means after the rite was performed. Nevertheless, the aforementioned performance of the rite is invalid, for a different reason: the shoe used was sewed with flaxen thread and, therefore, was never fit for this purpose. Since, however, the levir performed an invalid rite of halitzah, neither he nor his brothers are now permitted to marry the widow. Halitzah by the same levir being the only manner of releasing her, he may be forced, by persuasion or by flagellation, to repeat the rite with the proper shoe. For now the Mitzvah revolved on him alone, and we are permitted to scourge a Jew until he perform the required Mitzvot.
This Responsum is addressed to Rabbi Menahem of Würzburg.
SOURCES: Am I, 93, 94.