Which is better: for one of two people to stand in the first row and the other to stand alone in the second row, or for both of them to stand together in the second row, if there is no space for them together in the first row?

If two people enter the mosque and find a space for only one person in the first or second row, they should pray together behind the row, because if one of them were to enter, the other would be praying alone. However, if they find space for two people, they should both advance to it, as this is part of completing the rows, and neglecting it is contrary to the Sunnah. Their prayer is valid in both cases.