A house with a second living unit on the same water line is really two households drawing from one softener. The usual sizing question, how many people live here, gives the wrong answer when the basement apartment has its own kitchen sink, laundry hookup or bathroom. The tank does not know which unit a glass of water came from. It only sees the total load, and it has to regenerate on a schedule that fits that total.
Count the water use in both units
Start by listing every fixture in each unit that draws water: kitchen sinks, showers, tubs, toilets, washers, dishwashers and any outdoor hose bib on the softened line. Then note roughly how often each one runs in a normal week. Do the same for the second unit, and write the two lists side by side. A tenant who works away from home most days puts a very different load on the softener than a family that is home all day, so do not assume the second unit is light just because it is small.
Check the water heater and the meter
Find out whether both units share one water heater. If they do, the heater pulls hot water through the same softener path for both, and the combined use becomes the number that matters. Also check whether the second unit has its own water meter. A separate meter does not change the softener math, but it changes who sees the bill and who is likely to notice a problem first, which matters when you decide who checks the unit and who buys salt.
Work out the combined daily load
Your hardness reading from your own test is the starting point. Multiply it by the total daily water use you estimated for both units to get the daily hardness load the softener must remove. Then compare that figure with the rated capacity on the unit's specification sheet. If the combined load runs close to the rating, ask for a larger unit or a schedule that regenerates more often, not a unit that is merely in the right range on paper. The installer should show the arithmetic, not just a model name.
What to write down before you compare quotes
Bring three things to every installer: the number of people in each unit, the fixtures in each unit, and whether the two units share a water heater and a meter. Ask each installer to state the sizing basis in writing. If one quote assumes only the main household and another assumes both units, the prices are not comparable even if the models look alike.
Who pays for the salt
Decide this before the unit goes in. If the owner pays for the softener and the tenant benefits from it, the lease should say who buys salt, who refills the tank and who calls for service. Leaving it vague usually means the tank runs low during the busiest week for the upstairs household.
For a broader checklist on comparing providers, see the Missouri Water Softener Buyer's Guide.