Isn't the problem here, that you have 2 indirect heating source --> heating sinks? Thus, a delta T across each when they transfer heat. Say if your engine coolant got to 80 deg C, there's a temperature differential and the calorifier will only ever get to (say) 70 deg C? Then if its 2nd coil is employed as a sink instead of a source, it will only ever achieve 60 deg C?
It would be better plumbed so the engine coolant, a coil of the calorifier, and the domestic radiators are all on the same run. That's what we had in our old boat and it worked great. The only problem was, the length of run made the coolant capacity quite high, the original expansion tank wasn't big enough, but we eventually sussed it and fitted a larger one, which did the job.