Pretty much every 'how to' I've seen about making a boiler with an electric heating element has said to use two - but is that simply a 'speed everything up' issue? Would it work just as well, albeit a bit slower with just one? That would avoid all these electrics woes.. :hmm:
Putting two in is attractive since they are so cheap, and easy to fit, why not have that extra power available to you? :lol:
I think the safest way to use the two elements is to ideally have the plugged into separate circuits in the house.
If that means powering one from the downstairs ring main and one from upstairs, use a suitably rated extension, and if it's one that coils up, extend it completely as such a hugh draw through coiled up cable is going to generate heat (or so the warnings say!).
The next best option is to plug the elements into the same ring main, but into different sockets - and make sure nothing else is on that will draw a lot of current - washing machine/tumble drier/kettle/heater etc.