I think those sentences are a mixture of two sets of Q and A sentences;
だれがきましたか。
かとうさんといとうさんがきました。
in the past tense.
きたのはだれですか。
かとうさんといとうさんです。
in the present tense.
So the sentences in your textbook may not be strictly accurate grammatically.
However we tend to use those kind of mixtures and understand perfectly well what they really mean.