So, this is almost certain to repeat a billion questions already asked, but the answers and solutions on offer are so varied, so diffuse, and so uncertain that I figured I'd just as well add to the storm, in the hope that Google would sort it out some day. I speak, of course, of the dreaded Home App vs. Nest App quandary, and the added wrinkle of two thermostats for a two-zone HVAC system.
As anyone who has tried to make two Nest thermostats work on their own, connect to Wifi, and crucially, appear simultaneously on the Nest App will know, this is about as easy as brain surgery, and with a higher failure rate. You will know about being shunted from the Nest App to the Home App. You will know about being prompted to instal the Home App when you already have it. You will know about connection failures and the inability to find an existing Nest device, etc., with no real explanation for why that should matter at all.
As it now stands, we've managed to have our existing Nest Learning Thermostat stay connected to the Nest App and control the main living floor of our house. But adding a standard Nest Thermostat to the second floor, which is independently zoned, is heartbreak, and will only work on the Home App. So as it stands, we can control the main floor using the Nest app, both floors using the Home App -- unless you want to adjust the schedule of the main floor, in which case you need to go back to the Nest App. Oh, and a remote sensor will only work in Nest, but not Home.
So in a nutshell, Google's Nest ecosystem is a semi-functional cluster***k of mostly non-interoperable apps and devices that speak different languages, encourage day drinking, and grind souls into dust.
