"Dual" t stats as you wish for are [expensive] Dual headaches
simply set the tstat of the OB to say 55 or 60 and the coal appliance at your normal dwelling temp 65-70 or higher if you wish -- your hauling the coal
should you loose fire on the solid fuel heater thing -- the temp will drop in the house and the back up OB will gracefully and safely pick up the load until you address the drama.
think about it -- what if the fault is the dual tstat????
With a redundant system and separation - your chances of failure are cut significantly