Mischna zu Wajikra 18:8

Beth Shammai permits the rival wives [of a deceased brother to be married] to the [surviving] brothers; But Beth Hillel forbids them. If they have performed halitzah, Beth Shammai pronounce them unfit to [marry into] the priesthood, But Beth Hillel pronounced them fit. If they have married their brother-in-law, Beth Shammai pronounce them fit [to marry into the priesthood], But Beth Hillel pronounced them unfit. And although these pronounce unfit and these pronounce fit, Beth Shammai did not refrain from marrying women from [the daughters of] Beth Hillel, nor did Beth Hillel refrain from marrying women from [the daughters of] Beth Shammai. And in the case of all matters of purity and impurity in respect to which these pronounce pure and these pronounce impure, they did not refrain from preparing foods requiring a condition of purity each by means of [the vessels of] the other.