First thing I would try is turning one stat off or down as low of a setting as possible. Then go to other stat. Turn it up, make heat come on. Does it warm that side of house?
If so, turn temp down on this stat as low as possible. Go to other stat. Turn temp up. Make heat come on. Does it warm that side of house?


Had one where customer complained her bedroom stayed cold. She would shut her bedroom door at night. She also would leave fireplace logs burning. The logs, being a heat source, would warm the room thermostat was mounted in. Thermostat felt the room needed no heat, thus it would not call for heat. System that provided heat to entire house would not come on. Bedroom got cold..... I provide this bit of information as an example of sometimes it's the simple things.


If you want, if both systems work after performing above suggestion, choose the stat you want to control both sides of house. Find the boiler this stat controls. Should have a transformer. If this transformer is rated less than 75va, change it to a 75va. Take wires controlling stat you are removing and connect them to the new transformer, along with the wires from stat you are keeping.

You now have both boilers connected to one stat. I think this will work.

Hope this helps. You have helped me with a ton of information in the past, and I appreciate it.