if there's only one filler then yes, they're in parallel with an equalizer tube. Very doubtful one fills/drains into the other; actually the more I think about that the more I realize it HAS to be parallel, otherwise there'd be a pump between them. The indicator panels are not accurate at all. The tank sensors are placed on the side of the tank so there's still an inch or so of tank above the top sensor; that's why it says "full" but still takes more water.
Also the total water capacity advertised includes the water heater, assuming it is a tank type.