Apple and peach are my favourite fruit这句话中是两种不同种类的水果,所以用复数形式are
Bread and butter is their daily food这句话中按西方的传统观点,抹上黄油的面包已经是一种约定俗成的食物了,所以用单数形式.比如Knife and fork is the tableware in the West(刀叉是西方世界的餐具）
又比如That singer and dancer is my friend(那个歌手兼舞者是我朋友）That singer and that dancer are my friends(那个歌手和那个舞者都是我朋友）