My players came up with an interesting idea using the reverse gravity spell recently. At the time, I let them do it because it made sense and I like to keep things flowing - but I'm wondering whether the actual mechanics would work as I allowed it to.
Reverse gravity lifts everything centered on a point in a 50-foot up to 100 feet upwards. Two of the players in the party had taken the spell. On player 1's turn, he cast it at ground level to lift a group of enemies the full 100 feet; the 2nd player then separately cast the spell, centering the area of effect at the top of the previous spell's area and lifting the enemies another 100 feet. On player 1's next turn, he stopped his version of the spell, meaning that 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 for that casting.
At the time, I was unsure whether 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 one casting of the spell as they cast it a second time, lifting people several hundred feet into the air before then dropping them.
Can multiple castings of reverse gravity be used to lift the same target(s) several hundred feet in this way? Are there any rules for adjudicating this situation, or is it up to the DM to decide?