Thermostat settings two hvac units older house
You ask a tough question and I assume those are the articles you've read that left you with no definitive answer.

Personally, I've found it least expensive and of course most comfortable to set-it-and-forget-it in my last 2 homes...the only ones that had central air. By just maintaining the thermostat's comfort range of a few degrees the system will only activate briefly once or twice an hour typically.

Mine's set to 70 and only on the 95-plus days does it occasionally fall behind at mid-afternoon for an hour. It depends on cloud assistance here and there or wind being present to cool off the roof and siding.

In your case of 2 systems and 2 floors your only real course of action for any further experimentation or "construction" would be to install a door...likely just temporarily with even plastic sheeting taped liberally at the top and bottom.

A door or the enclosure of the top of the stairs (so cold air doesn't fall from upstairs) would allow you to see if turning the upstairs off or up so high during the day is beneficial at all. I'd suggest 74 or 75 all of the time, unless you're actually comfortable at the 78 for the majority of 24-hours.