All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The majority of Muslim scholars are of the view that a worshipper's Prayer becomes invalid if his place of Prayer is in front of his Imam.
One should not be ahead of his Imam while performing the Prayer. Thus, it is your duty to fix a line for the first rows on the 2nd floor so that all followers line up behind their Imam, or you may ask the Imam to lead them from the second floor. However, it is better for you to have a mosque where all people pray on the same floor, since praying on more than one floor is disliked according to the majority of scholars; otherwise, there is no harm.
Allah knows best.