between后面用单数和复数都可以,但是,用单数的时候往往会同时用and,构成beween A and B结构,例如:

This plant blooms between May and June.这种植物在五六月间开花(摘自《柯林斯英汉双解大词典》)

There was mutual mistrust between the two men.两人之间互不信任(摘自《柯林斯英汉双解大词典》)