We have a gas water heater (a Rinnai) in one of our caravans. It didn't specify any type of gas but there might have been something about pressure - I leave that sort of techie thing to Spouse
OK, I realise that our water heater/space heater/cooker/fridge/lights combination won't have the same capacity or output as your French cottage but it works well with one bottle of butane or with one bottle of propane in most ambient temperatures. I've never, ever, heard of any appliance needing two bottles in parallel! For what you suggest to happen your rate of usage must be extremely great. A daughter has used one large bottles of butane in Wales for fuelling her very large cooker with no problem. In winter she switches to a bottle of propane - and I think she's considering year-round propane.
Perhaps the French are over-cautious?
Mary