Warm Air rises - even when you cool the lower floor - the warm air will rise. There is a temperature coefficient for how hot your 1st floor ceiling is versus the temperature setting of the upper floor. In well designed systems your cooling air from upstairs will draw down to the lower floor - so in effect the upper AC unit would be cooling that air too to some degree.
One effect is this : Your upper AC unit was sized for that floor - it will run to satisfy the upper temperature setting.
While saving energy is good your upper unit will be working harder.
What the trade-off point is (1st Floor Temp vs 2nd Floor temp) vs Ambient outside temp along with the Air circulation flow of the home from 2nd floor to first floor will all factor in to the threshold of energy savings vs A/C unit life.
Now all of that said knowing the actual values to plug in - most are easily obtainable - air flow 1st floor to 2nd floor is more difficult to know.
Then you would need to do some calculations based on that data to determine the optimum 1st floor Thermostat value vs 2nd floor Thermostat value.
Given your variance of 6 degrees - I think you are probably very close to a good setting if Air flow 2nd to 1st floor is ok and the units are sized appropriately - for example if your 2nd floor unit is not undersized given the space (Volume of Air to be cooled).
Just as an FYI : Your 1st Floor unit will also run - just not as much, depending on how much airflow makes it to the second floor or heats up the 2nd floor - flooring will determine how much the upper unit runs. This Air Flow equation is your big unknown value.
Hopefully you have continuous soffits with a continuous ridge vents..