Single tank or dual tank water softener: what continuous demand actually means

No. Water quality out of either configuration depends on the same resin type and capacity, not on how many tanks it is split across. A dual tank system does not soften more thoroughly, it only keeps softened water flowing during the brief window a single tank would spend regenerating. If the goal is better softening rather than continuous availability, resin capacity and correct sizing matter far more than tank count.

Almost never. A metered single tank valve schedules its regeneration cycle for the lowest demand period it can detect, typically the middle of the night, and the cycle itself generally finishes in well under two hours. A household asleep during that window has no practical reason to notice reduced flow or a brief bypass, which is exactly why dual tank rarely earns its extra cost for a typical residential household.

A low recovery well is one that refills slowly enough that it cannot supply water fast enough to both run a softener regeneration cycle, which uses a meaningful volume of water on its own, and meet ordinary household demand at the same time. A dual tank system solves that specific conflict by taking one tank offline to regenerate while the other continues supplying the house, which a single tank cannot do.

Not automatically. Bathroom count and family size affect how much resin capacity a household needs, which is a sizing question answered by grain capacity, not by tank count. A large family that sleeps overnight like most households still loses nothing to a single tank regenerating at night. Continuous, round the clock demand, not household size, is the actual qualifying condition for choosing dual tank.

Generally no, not by adding a tank to an existing single tank valve, since dual tank systems use a different control valve, such as a Fleck 9100SXT, built specifically to alternate between two tanks. Moving to dual tank later usually means replacing the whole system rather than adding a component to the one already installed, which is worth factoring into the decision up front if continuous demand seems like a future possibility.

Total salt use is driven mainly by total grain capacity and how often regeneration happens, not directly by tank count, so splitting the same total capacity across two tanks does not automatically increase salt consumption. Two smaller tanks regenerating on their alternating schedule can use a similar total amount of salt to one larger tank regenerating less often, and the manufacturer published salt dose per regeneration for the specific valve is the number that actually determines it.