If two people intend to pray, should one of them pray behind the the Imam with a mentally ill person, or should both people pray in congregation and disregard the mentally ill person?

Intellect is the basis of religious obligation (taklif). Thus, one who does not comprehend proximity [to God], such as the insane person or the mentally deficient, is not held accountable. Their presence in the row is not considered as their absence. However, one who comprehends prayer is considered to be in the row and stands with the second person behind the Imam, forming a row behind him.