Is it possible to lift things multiple times with different castings of reverse gravity?

My players came up with an interesting idea using reverse gravity recently which at the time I let them do because it made sense and I like to keep things flowing but wondered about the actual mechanics of it.

Reverse Gravity lifts everything centered on a point in a 50ft circle 100ft up. 2 of the players in the party had taken it, in player ones turn he cast it at ground level to lift a group of enemies the full 100 feet, the 2nd player then cast the spell a second time centering at the top of the previous spell and lifting the enemies another 100 feet. In player ones turn he stopped his version of the spell meaning player 2 could then drop them 200 feet. As there was nothing to grab hold of on the second casting there was no saving throw

At the time I was unsure if the initial casting of the spell would lock them in at 100 feet but decided to go with it. My players have also asked me if one of them could cast it repeatedly turn after turn. Effectively cancelling it as they cast it a second time lifting people multiples of 100 feet into the air before then dropping them.

Is this a DM decide situation, which I am fine with, or are there any rules stated anywhere for this situation?