My previous post was deleted by the Community. I'm incorporating very helpful comments from Stack users here in this updated entry.
I have hydronic baseboard heating that uses a Weil McClain oil boiler. Two zones, one for the first floor and one for the second. I have central A/C with the air handler in the attic and the condenser outside. The A/C also has two zones, one for each floor.
I have two thermostats, one on each floor, with OFF, HEAT, COOL, FAN ON, FAN AUTO options. Each thermostat controls both heating and cooling. My first floor thermostat is currently a Honeywell T8775X (digital round, non-programmable). My second floor thermostat is a Honeywell CT87 (classic round, non-programmable).
Please see included images of A/C, boiler, and thermostat wiring in link above.
I plan on installing two Ecobee3, one for each zone. What would be the best way to connect the Ecobee3 - to the A/C or to the furnace? Is it ok to connect both thermostats to one system (i.e., both to the A/C or both to the furnace)?