I have 2 Ecobee Thermostats. I can't seem to add the 2nd one. How do I add that accessory

Hi RW, We appreciate you reaching out to us. We understand that you are experiencing issues adding your second Ecobee Thermostat to the Home app.

We’d like to gather additional details that may help us be in a better position to assist you.

Are you receiving any error messages?

Does it help if you remove the first thermostat, add the second thermostat, then add the first thermostat again?

In the meantime, we recommend that you make sure that you check for a software update.

While it isn't inherently assured that an update will resolve the issue, updating to the latest software version for your device can help isolate potential software issues. 

Kindly secure a backup and update.

How to back up your iPhone, iPad, and iPod touch

Update your iPhone, iPad, or iPod touch

Take care!

I am also having the same issue and it is not an update issue with either my iPhone or ecobee device as both are up-to-date. The first time I attempted to add the second thermostat, I received the attached error. When I reset the second thermostat and re-attempted to add it during the WiFi process, HomeKit deleted my first thermostat. When I added the first thermostat back, the second thermostat became non-responsive in HomeKit. This seems like a software bug; HomeKit is unable to handle multiple smart thermostats in a single home.