No disresepect to anyone here, but what possessed the plumber who
overengineered our replumbing (before my time) to put two water tanks
in the loft, linked together low down and fed in parallel by two
ballcock valves? Apart from the obvious two pointsof failure for no
apparent advantage, it strikes me that due to the imbalance one of the
valves will get stressed as the other fills to a higher level and so
the rubber diaphragm seal will fail earlier.

Regardless of the wisdom or otherwise of such an approach, when said
plumber failed to use any solvent weld on any joint when assembling
the overflow, causing the pipe to drip whilst we were on holiday and
so damaging the plaster and decoration it makes me remember why I
prefer to DIY [Rant mode off]

Pete