everyone gets it George. We know it would "work", but your missing the point. it does not comply with gas safety regulations
When will you get that into your head?
I will spell it out for you. Read carefully. All gas appliances connected to a Gas supply must recieve adequate inlet pressure when operated at the same time
How you wire them up is completely irrelevant as the Gas Safety Regulations do not cover this kind of work. Some numpty could come along and unknowingly change the switch and make both boilers dangerous
The Rinnai Twinflow is an approved appliance. You would have to go into the boilers electrics and modify the appliance to overide the switching. That would mean you are modifying a gas appliance which is illegal.
Re-wiring a simple switch would not be illegal. Which is why this method would not and is not approved.
Your advice is dangerous as always.
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