I don't quite get it why they can't take another LLM and vet the output of the first with the second one. Surely they would not have the same hallucinations and

I don't quite get it why they can't take another LLM and vet the output of the first with the second one. Surely they would not have the same hallucinations and would be able to detect hallucinations of the earlier LLM. Maybe it would cost too much in terms of tokens? I don't know but I would expect it to be relatively easy for an LLM to detect "hallucinations".