The validity of a pure person praying behind an Imam who is leading two people when one of those two followers is impure (lacks ritual purity).
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
If the Imam leads two individuals, and one of them is not ritually pure, the pure individual should still follow him in prayer (i'timaam). This ruling is understood to apply if the person who became impure (muhdith) is aware of their state and leaves the prayer line. If the impure person leaves, the remaining pure person joins the line, positioning themselves to the right of the Imam if they were not already there. If the impure person remains behind the Imam while being aware of their impurity, the prayer of the Imam and the followers is invalid.
Supporting text
If the person who became impure was unaware of their state of impurity until the prayer was completed, the prayer is considered valid. This is because if the person had been the Imam, the prayer behind them would have been valid; thus, following them while they are unaware is even more certainly valid.