If humidity control is important to you than you might want to only have one
unit operating at a time (unless it's a VERY hot day).

Use the scheme you outline but make the set back temperature of the "other
unit" high enough that it usually doesn't come on at all when in "set back"
mode. This will may be non set back unit operate almost continuously which
helps control humidity.

You might pick a set back temperature of up to 85. This isn't super
uncomfortable if you have to go into the hot zone to get something. You
will have to experiment to see how long it will take to bring that zone down
to 76F while the downstairs unit is also on. The goal is that you shut
down the downstairs A/C just before the upstairs is cooled to 76F.