As someone that has lived in the electrical industrial world for a very long time and play with HVAC, I would just use an interposing relay. The thermostat runs the relay coil and 2 independent relay contacts run to each boiler.

Now is that the best solution to run both boilers off the same thermoset, I don't know. But this will definitely allow you to run both boilers off the same thermostat with no issues.