The latter. It would be impossible for one user to usurp power from the other until the first user has turned off their isolator with a simple 2 contactor system. An interlock system still isn't ideal, but most people would probably prefer not to have to pay for a supply upgrade.
As per the drawing I did above. Agreed, it's not ideal but it overcomes the need for two large electric showers in one property. God knows why somebody would want that arrangement though.