Two people entered the mosque and there was only space for one person left in the front row

If two people come in and they find that there is only enough
room for one man in the first or second row, then they should stand
together, because if one of them joins the row the other will be left
standing on his own. In this case it is better for them to pray standing
together behind the row. But if they find enough room for two men in the
row, then they should both stand there, and not stand behind the row on
their own, because that is contrary to the Sunnah. The Prophet (peace
and blessings of Allaah be upon him) encouraged completing the rows one by
one, starting with the first. But if it so happens that they did that, then
their prayer is valid because neither of them stayed on his own away from
the other. End quote.