it's repeated that two schools both of witch are b

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因为后面的句子必须是前面的定语从句,你也可以用them但是要这样用
it's repeated that two schools,and both of them are being.
======以下答案可供参考======
供参考答案1:
定语从句的考察，which在这里引导英语从句，先行词是schools，在从句当中作are being的主语，若是they，那么就是两个句子，如果没有连词是不成立的，中间加个and也可以，那么就是两个并列句了