[T77911S]

this may be why most 2 stories have 2 units.
a better location for the T stat.

have you checked the insulation in the attic or added any? you may even look into the spray foam stuff, i have it in my music room. they can spray it on the rafters of the roof and it will drop the attic temp a lot in the summer. the heat from the attic will radiate into the house late into the evening.

i would think you could parallel 2 controls, one up, one down as long as the 24v power source is the same.