At the restaurant I had X and Y. They were out of this world!
At the restaurant I had X and Y. It was out of this world!
X and Y are two separate dishes. Now in the first sentence, the usage of they seems fine and correct. But in the second one, the usage of it to describe two different dishes does not. But, if it, instead of meaning, two dishes, meant the whole eating experience, then that makes the sentence grammatically correct in regard to the previous statement again, right?