Why are you doing it this way anyway? Because you happen to have tanks or are you keeping then seperate due to chemical use?
If first option, sell them and invest in one tank!
Alternatively, are you stacking them on each other (with support) or side by side (infront of the other)? Simply feed tank a into main tank b. So as water level drops, its automatically starts to fill. But maybe incorporate a tap so you have control (for whatever reasons your doing this)
Whats your plumbing skills like?