Assuming that it is regarded as a single installation (including the E7 heaters) then BS7671 also requires that 462.1 & 462.1.201).And they do to remove ambiguity. One of my agents insists on a single main switch for total dwelling isolation, including E7 heaters.
I suppose the former 'does no harm' (other than to create an unnecessary cost) but, as has been discussed, the latter could, in some circumstances, result in an increase in potential danger.Another agent [not one I employ] insists the gas & water etc bonding is done twice and any device sitting within unions [think heating pump, water meter etc] has a bond across the pipes
One has to wonder how it comes to pass that a letting agent has enough expert electrical knowledge for it to be appropriate for them to require things which exceed the minimum required to satisfy BS7671. ... but. given that they have gone down that path, one also wonders why have they stopped at requiring the bonding to be duplicated - they might also have required that CPCs, MCBs and RCDs/RCBOs etc. be 'doubled up'!!
Kind Regards, John