当宾语从句不止一个时，第一个that 可省略，而其他的that不能省。不然会引起歧义，让看句人认为是主句和从句并列了。

I think (that)it will clear up this afternoon and that they will come to say goodbye to us.

我认为今天下午天会放晴，他们会来向我们告别。

He says (that) they will come back in a week and that he will call me when they come back.

希望能帮到你。