We have a tri-level house in SC with 2-zone A/C - upstairs A/C is electric, downstairs is gas (is that only for heat?) We thought we were smart by moving our bed downstairs to the coolest part of the house & turning off upstairs A/C & closing doors to bedrooms. Electric bill for June-July was $100 more than May-June. Would we be better off just running both A/Cs & keeping settings @ 75 in daytime & 70 @ night?

Not sure you have it right. Gas AC units are very rare and mostly used in commercial buildings. How does chilled air get down to the lower level? Sure it isn't a zoned system? How many thermostats and outside condensers? Why not just leave it at 75 and put the fan on to run all the time? 70 puts a big load on the AC and in very hot weather 95 or so, will not be able to reach the set point.