Jeff, to clarify: Sometimes in well houses there is a main line and then it branches off to other lines that can be shut off individually. In my home's case since we have a workshop & barn as well as hose bibs out in the field we had ours split 3 ways. One went to the barn/workshop, another went to the house, and another went to the back fields/garden. Granted, I've got almost 32 acres so I have a lot of area to cover. For some homes they may just have two branches-- outdoor & indoor. It's smart bc if you get a break in an outdoor line you can shut off the water to the outside but still have water on the inside the house.
Having separate lines for indoor & outdoor can help if outdoor pipes freeze or if someone trespasses & runs a hose from your outdoor faucet to their trailer to steal your water (both of which happened to us-- shutting off outdoor water resolved it).
So, the OP could set up a softener just for the inside lines. I wonder if PEX could be used for a water softener to make it a bit easier to route things.
Editing to add that while it rarely gets below freezing where I live, it did start to get colder in recent years so we added a small ceramic heater (rated for outdoors) to the well shed- so you can have a heater. You can also have insulated walls to keep it warmer. So long as the lines inside don't freeze, water softener should work. Since many of them work using salt, I've heard they tend to corrode the anode rod in the water heater faster though.