Is this a comfort question or an operational cost question? You probably will need to experiment. I would leave the upstairs HP off and run the downstairs unit only. Check your meter and get a reading. Check it again in 24 hrs. Get a reading. Then do same with the upstairs HP on at a lower setting. Make the comparison in usage. Only you can judge for comfort. I will assume there is not a door that separates the two floors/zones.

IMO

I don't think it will make much difference because heat rises. So the upstairs will always be this way, the same thing happens in the summer... cool air falls and heat rises because you don't run the AC during this time the effect among thermostats is a much wider temperature range between the two floors.

As far as how hard a system has to run, this really is more of a figure of speech. Probably as high as 98% of all air conditioners / heat pump combos they are single speed machines (residential market).

Basically this means the machine is either on or off. If the system is on it produces the amount of cooling / heating it is designed for.

This doesn't change given the load, because the machine is only capable to produce the number of BTU's it was designed for given the load.

In winter you have to understand what a heat pump is doing... it is taking the warmth in the air outside and transferring it to the inside. What warmth? It's cold outside!

Again these things are figures of speech. The problem is that when ambient temps approach 32 degrees outdoors the coils start to freeze up forcing the heat pump into defrost and cycling your resistance heat inside or whatever you have for emergency back up when the heat pump can't deliver.

If the heat pump is in defrost, it is not transferring any heat during these times to the house. This is where you may get the notion that the heat pump is 'working hard'.

But if the heat pump is on, the amount of work that is done is virtually the same. The difference is the load and conditions in which you are placing upon it to perform.

Not a comfort question but one of efficiency. In summer, we only ran the downstairs unit to cool the house. Seemed very efficient based on electricity costs. We're new to heat pumps, two-story separate units, and Coastal NC. I hear it rarely gets below freezing here. There is no door to close off the upstairs. I'm not worried about freezing pipes. I just wanted to know if there is a 'best practices' rule of thumb for how to deal with a 2nd floor that isn't used in a dual heat pump situation.

Coastal NC wont get that cold (at or below freezing).

Not sure about a 'rule' per se, but I agree with Mike in that you could/should set to a lower temperature and leave it there.

I'm not worried about freezing pipes. I just wanted to know if there is a 'best practices' rule of thumb for how to deal with a 2nd floor that isn't used in a dual heat pump situation.

It's not going to hurt it to leave the up stairs off. If the down stairs is used to heat, the heat will rise and heat the upper level.

If you decided to set the upstairs unit to heat, depending on out door ambient / load conditions it may never turn on anyway because if you are running the lower level unit and the heat rise rule may keep the 2nd floor above the temp in which it would turn on to heat.

Now if things were flipped: let's say you spend all your time upstairs and you decided to shut off the lower level. In the summer during heavy heat season the upstairs unit under extreme conditions: like 5 to 15 degrees above your location 'DESIGN' temperature, would probably not keep up because of the heat rise rule and the down stairs unit being off.

Same situation in heating season: won't matter because heat rises.

Your heating and cooling system are used for comfort. For the most part you can set the thermostat to any setting you want. Some air conditioners have recommendations not to run below 55 degree OAT (requires specialty equipment) The same can be said of some heat pumps do not want you to run in heating mode with OAT above 65 degrees. These 'finer detailed' things vary from one manufacturer to another.

(Due to frozen water pipes conundrum, you can subdue this with a minimum 55 degree heat setting. This way the system will stay off except in the event of really cold weather.)

probably for OP's location, the upstairs HP system is more important for cooling than heating...

IMO

The winter design temperatures for North Carolina range from 15 to 28 degrees F depending where you live. Let's say your area is 25 degrees. That means on a very cold winter night you could be in the low 20s. You may not realize it but there could be a water supply pipe installed in an exterior wall between the sheetrock and the exterior insulation in one of the upstairs bathrooms. Normally this is not a problem under typical indoor and outdoor temperature conditions. But it is possible for a pipe to freeze if the heat from the lower floor can't keep that pipe warm in the wall.

If it were my house I would set the heat at 55 degrees and not have to worry about it.

Yea ideally it would be good to run HP on 1st floor for primary heating and HP on 2nd floor for primary cooling.

Since you dont use the 2nd floor you probably dont even need to cool it when not in use honestly. Up to you.