This works with some coordination.
Caster 2 casts their reverse gravity 5 feet below the top of the first caster’s cylinder. Caster 1 drops their concentration. The creatures are now only affected by one reverse gravity, the second one, and will now rise 100 more feet.
Notably, the only limitation given on where reverse gravity is cast is “a point within range”. It need not be cast on the ground.
The important piece here that seems missing from your description is that caster 1 must drop concentration for the second spell to take effect.
The rules for combining magical effects say:
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.
Only one effect applies at a time, and the only guidance for which applies is “most potent”. Since the spells are the same, only one can apply at a time, and since neither is more potent, there is no reason to change which is applied. The first applies until it is ended.