Hi @vitamincm ,
Will the both be able to use the sensors? Or will I have to unpair/repair them every time seasons switch?
No, ecobee remote sensors cannot be shared natively between 2 ecobees.
However, if you use My Ecobee Bundle, you can expose all your ecobee remote sensors to SmartThings, and the zoning smartapp (ecobeeSetZoneWithSchedule) could use them based on the concepts of rooms/zones/schedules. In fact, you can use any ST connected temp/motion/contact sensors in the zoning smartapp; you’re not limited to the ecobee’s remote sensors.
(Not in your case) You can even control both ecobee thermostats (if needed) within the same zoning smartapp. One ecobee needs to be elected as the master and the other one will be considered as its slave in terms of thermostat mode and setpoints (with a temp offset or not). Each room or a group of rooms can be associated to a Virtual Zone Device for easier control.
Will one thermostat be able to use the other thermostat as an additional sensor? (In winter, the heat thermostat can use the AC thermostat to get temp and motion readings in that area. And vice/versa)
Yes, with My Ecobee bundle, this is possible. The ecobee thermostat can be exposed as an ecobee remote sensor, be associated to a room for use in the zoning smartapp as temp/motion sensors to enable similar features like the ecobee’s follow me and smart away.
N.B. Natively, one ecobee cannot be used as a remote sensor for another ecobee.
For more details about My Ecobee device DTH and the zoning smartapp, please refer to the ST community wiki (i.e., supported commands & capabilities,configuration, troubleshooting, etc).