I have two five amp lighting circuits, notionally upstairs and
downstairs but because of a funny house layout they overlap a bit.

I just put in a new stair light circuit and, for convenience, changed a
single gang light switch for a two gang one to cover the new circuit. It
was only afterwards that I realised that left me with one switch on the
"upstairs" circuit and one on the "downstairs", so you need to throw two
breakers to ensure that unit is dead.

While I am not going to lose any sleep over this, I guess it might not
be compliant with current regs, can anyone confirm? I have a relatively
ancient CU with all circuits protected by one RCD so it is fairly safe
against isolation failure.