Why not take 2 smaller storage heater, configure 1 washroom with 1 storage, and other one share with two washroom (or 3 storage with 3 washroom).  This you don't need more time to heat up the water (If you are the type don't use all the Washroom all the time), and you run more efficient because it heat up smaller amount water, and one bathroom don't need that much unless you want to fill up Jacuzzi.