If two people enter the mosque while the Imam is bowing, and they find a gap in the row that is only large enough for one, should they both line up behind the row to catch the rak'ah, or should one of them enter the row and fill the gap while the other waits for someone to line up with him, with the possibility of the rak'ah being missed by the other?

Scholars hold different opinions regarding an individual praying behind the row, and the most preponderant view is that it is permissible due to an excuse, such as the absence of a gap. The preferred action is to fill the gap in the first row. If one does not find a place, they pray behind the row. If someone comes and stands with them, there is no issue. Otherwise, their prayer is valid according to the preponderant view, because they are excused. If they choose to pray together to avoid the difference of opinion, there is no harm in that.