句子的先行词是主句的主语 The first house and the second house，in both of which 是复杂关系代词（引导非限制性定语从句），它与 in which 的作用是一样的，只不过因为有两个 houses，所以加上 both of 就变成了in both of which：

The first house and the second house are well-suited to their everyday situations, in both of which the kitchen is bright and spacious. 第一所房子和第二所房子都非常适合他们的日常生活，两所房子的厨房都明亮宽敞。

类例：Mrs Strachey and Mrs Montague were my husband's friends, with both of whom and their households I became acquainted soon. 斯特雷奇太太和蒙太古太太都是我丈夫的朋友，我很快就认识了她们两人以及她们的家人。