When one tank is physically taller than the other, You would have to block the flow to the shorter tank so it does not over-fill.
Having an equalizer line 2" or 3" between the two will help the shorter tank over-fill unless your top lid is air-tight which they hardly ever are or we all would have problems using them for pressure washing when pulling water out of them. Have you ever noticed that even with the top lid tight, air still gets into the tank so you can keep on pulling water? If it was air tight, the pressure washer would starve for water eventually. Because of this air leak, the tank will over-flow from the taller tank via equalizer line once the taller tank water level is higher than the shorter tank.
To have the best of both worlds, you might have to put a ball valve on the equalizer line (to block the shorter tank when full to keep it from over-flowing) and possibly a couple ball valves to the fill lines, think of a "T" with a ball valve on each of the shorter sides so you can block off the shorter tank while keep filling the taller tank. Block in the equalizer line when the shorter tank is full or keep it blocked and just use the fill valve to the shorter tank when the tank is full then when the taller tank is full, just block in the meter or hydrant.
This way you can fill both tanks all the way and not over-fill them and when the taller tank is level with the shorter tank, open the equalizer line so you are pulling out of them equally until they are empty. With garden hose lines filling them both or just one of them, the equalizer line will keep them level.
This is what I would do if I was in that situation which I might be before the year is over for a particular use.
Hope this helps