I have got two tanks...
And what I,ve done is, I put a hole at the top of both tanks (One slightly higher then the other) and pushed the hose through so when the first tank fills up then the overflow goes into the other tank...
It take about 4.5 hrs for one tank to fill up with a booster pump so I set the timer on the switch for 6hrs and when it's finished cos I dont have a stop on the outside tap to stop the water it goes on the slow fill so in the morning I have two tanks filled up.
(I must say the both tanks avg about 30 tds)