When two households share one house, the softener is usually sized for the household that was there when the water system was first looked at. The second kitchen, bathroom, and laundry often get added to the picture later, or not at all. Before you accept a size, count both households as separate water users.
Start with the fixtures, not the headcount
Headcount is the first number installers ask for, and in a split household it is the least useful one. Two people in a basement apartment with one bathroom and no washer draw very differently from two people upstairs running a full laundry day. Walk each unit and list every fixture that uses water: showers and tubs, toilets, the kitchen sink, a dishwasher, an ice maker on the refrigerator, a washer, and any outdoor hose bibs or irrigation on the same line.
Find out where the lines split
The softener has to treat water before it branches, or each unit needs its own treatment point. Ask the installer to trace the main line with you: where the cold supply enters, where the water heater connects, and whether the second unit draws from a branch that comes off before or after the softener. If the second unit is fed from a line that bypasses the softener, its fixtures will keep seeing hard water, and the scale or spotting you wanted to fix will continue in that unit.
Check the bills and the water heater
If both households share one meter, the bills show combined use, which is useful for sizing. If there are separate meters or a split account, get the figures for each one. Sizing from several months of real bills, including the heavier months such as holidays or when a relative stays over, is more reliable than a rule of thumb. Also check whether the water heater is shared. One heater serving both units means the softener protects it for both, so the sizing has to cover both.
Size for the busiest day, not the average
A softener runs short when several fixtures draw at once, such as a morning shower in one unit while the other runs a dishwasher and a washer. An average day can look comfortable while the combined peak day does not. Ask the installer to size for that peak day and to explain what happens on it. A unit sized only for average use will be undersized in exactly the weeks when both households are home.
Decide between one system and two
There are two layouts to compare. One whole-house softener at the main entry covers both units with one set of plumbing, one salt schedule, and one service visit. Two separate points of treatment give each unit its own capacity and its own shutoff, which can matter if one household is away for long stretches. Ask each company which layout it recommends and why, and ask for the reasoning in writing: the fixtures counted, the peak day used, and the capacity chosen.
Questions to ask before you choose a size
Ask the installer which fixtures were counted for each unit, and whether the basement, guest suite, or in-law space was included. Ask what peak-day scenario the recommended size is built on. Ask where the softener sits relative to the branch that feeds the second unit, and what treatment, if any, that unit gets without it. Ask whether the regeneration schedule suits the household that is away most of the week. Get these answers on paper before you compare prices, because two quotes sized for different households are not comparable.
Settle who pays before install day
Agree on the cost split before the unit goes in. Decide who pays for the softener, the salt, and any service calls, and who approves a repair. This is a household agreement between the people involved, so put it in writing and keep a copy with the install paperwork.
When you shortlist installers, the company ratings on this site show how each company was scored, which can help you narrow the list before you ask these questions.