Nice idea but unfortunately it doesn't work that way.

To make matters worse the Nest isn't recommended when considering HVAC zone systems. These T-stats are power stealing, which pose problems for many a system with just one. I would say if you live in a mostly moderate climate (light on cooling needs & light on heating needs) then the Nest would probably be ok for quite some time. The HVAC system wouldn't run that much and because of that, the nest would have ample time to steal the power it needs to be so fancy looking. For zoning systems I would never choose the nest.

Without zoning (control board & dampers) even if you could get the two thermostats to operate the equipment you'd be forced into the same problem you'd have with one thermostat.

Too hot or too cold areas, without zoning there is nothing to shut air flow off to areas that aren't cooling or heating for that matter.

If you haphazzardly shut off air flow with no real place for that air flow to go... blowing apart duct work, destruction of your HVAC system are likely results.

HVAC zone systems are not created equal. If you thought the thermostat market was bad at creating garbage gadgets, the zoning network of garbage controls, dampers and so on.... so you consider this sort of thing too cost consciously and in as little as a few years you may find it was all a waste.

If you want to dig further at this with solid information click this link to one of my websites, as I have covered this HVAC zoning topic quite extensively including vidoes that were made long time ago.

I service the Katy, Texas area.

For your situation, a Nest temperature sensor would work and cost much less than trying to get to Nest thermostats to work together. Here is a review done a few years ago: Nest Temperature Sensor Review: Almost Great

The sensor is supposed with work the Nest 3rd Gen. The biggest compliant in this review was the limited scheduling. Check to see if there have been any improvements. But form what I say on the video it seems it would work well when you want to prioritize the temperature of one floor versus the other.

But form what I say on the video it seems it would work well when you want to prioritize the temperature of one floor versus the other.

Unfortunately if you have no method of shutting air flow off to the opposing zone, you will still have the problem of "over cooling or over heating" the opposing zone.

A sensor is not going to magically average out temps with one ac and two floors needing conditioned air. (heat rises, so these so called sensors will be fighting themselves.)

A sensor in one area is not going to stop the flow of conditioned air to an area in which the sensor is not calling for conditioned air. The gadget market is a billion dollar market... because people like to think there is some simple fix to what is normally considered a complex problem.

a two story structure needs more than a gadget to have any real chance of working. Definition of working: Regardless of outdoor ambient conditions, you are comfortable in "all areas" of your home.

Anything less than that? Fill in the blank here_____________.

Why not install just one and use the Nest phone app to change the temperature when you are on the other floor?

You could do that, but you would have to calibrate the second floor temperature with that of the first floor. We don't know the location of the HVAC equipment, but it is likely the second floor is going to be warmer in the summer, and colder in the winter. The sensor eliminates the calibration error for each season. It is the best you can do without installing a zoned system and it only costs $40.

The sensor eliminates the calibration error for each season. It is the best you can do without installing a zoned system and it only costs $40.

If you honestly think $40 is going to solve this problem to any large extent --- afterward you will start realizing more clearly of who to listen to and who to put on ignore.

You could get the same failed problem by lowering or raising the temperature on the thermostat without the sensor. (Keep the $40 in your wallet, it's a waste.)

This is why the billion dollar gadget market exists. A fool and his money are soon parted.

I have only one caveat with anything I recommend or sell to someone else: It will work.

My definition of "work": hot out door ambient temp, mild out door ambient temp, chilly out door ambient temp, cold out door ambient temp.

"yes but what you charge is more than $40?" -- you get what you pay for and sometimes alot less. This is why you use a pro, as it were.

Anyone can do HVAC, that doesn't mean they should.