it’s reported that two schools ,both of which are being buil

it's reported that two schools ,both of which are being built in my hometown ,will open next year
both of which
为什么不用both of them?

把两个逗号中间的句子去掉以后,头尾两段形成一个完成的句子.这说明,中间那段不是句子,是一个从句,定语从句,修饰two schools.定语从句有定语从句的用法,two schools是先行词,which是引导词,因为有both of 这个介词短语在,所以不能用that.扯远了,你可以找本书详细研究下定语从句.