Do you not have a changeover valve somewhere so you can choose which tank is feeding the c/w system? The valve on Playtime is in the cupboard behind the shower, where the water pump, shower drain pump and filter are located. The output pipes from the 2 tanks feed through to a 2 way valve – lever vertical is front tank, horizontal is rear tank? The out put from the valve goes direct to the c/w pump.
Surely you have something similar, otherwise how do you change from one tank to the other?