Storage Heater Size

[xucclf 0]

Hi guys, I am getting a storage heater for my resale flat, my common bathroom and master bathroom are separated by service yard and master bedroom. I am going to put rain shower in both bathrooms. Me and wife will be using the master bathroom toilet, but we intend to rent out a room so the other bathroom will be for our tenant.

Will like some advice on whether I should get 1 x 25L, put in one bathroom and pull piping to the other, or get 2 x 15L for convenience?

My concerns are whether it may take too long to heat up a 25L heater, and will there be enough pressure to run the water to the other bathroom's rain shower?

Also, is it possible to have two power switches for a single heater? cos I don't really want to have to run to the other bathroom to switch on heater for bathing.

Thanks for advice!