Bathroom water heaters

That sounds like one option. Although 40+A change-over switches are neither small nor pretty, and probably isn't that convenient (too easy to forget to switch back after a shower, so no hot water elsewhere). An automatic  'shower priority switch' (a.k.a. a load shedding relay) is sometimes a neater solution if the supply isn't meaty enough for both (e.g. www.meteorelectrical.com/.../garo-1-row-shower-priority-metal-board.html )

Depending on the supply and other loads you might be OK just wiring both in normally (there are plenty of domestics with two or more instantaneous electric showers on a normal single phase supply which have given years of trouble free service, regardless of what the OSG might say about diversity).

Or these days of pressurized hot water cylinders, a simple immersion arrangement (especially one that can do the bulk of the heating from an off-peak supply) can be a quite workable solution, if you have the space.

   - Andy.

That sounds like one option. Although 40+A change-over switches are neither small nor pretty, and probably isn't that convenient (too easy to forget to switch back after a shower, so no hot water elsewhere). An automatic  'shower priority switch' (a.k.a. a load shedding relay) is sometimes a neater solution if the supply isn't meaty enough for both (e.g. www.meteorelectrical.com/.../garo-1-row-shower-priority-metal-board.html )

Depending on the supply and other loads you might be OK just wiring both in normally (there are plenty of domestics with two or more instantaneous electric showers on a normal single phase supply which have given years of trouble free service, regardless of what the OSG might say about diversity).

Or these days of pressurized hot water cylinders, a simple immersion arrangement (especially one that can do the bulk of the heating from an off-peak supply) can be a quite workable solution, if you have the space.

   - Andy.