I use the on-demand meter set like 1/2 of a twin.
On the twin it changes from tank A to tank B when the meter gets to 0, but a single set up on the regen when meter gets to 0 works just fine when water is going from well to softener to holding tank, the twin could be used but it is over kill in that case.
There is a list that Craig as out, there might still be one in Mesa... a Kinetico..
All twin tank type softeners regenerate while you are using water, normal softeners do not. Both types meter the water usage (count down gallons) and when the meter gets to zero on a twin the control valve swaps the in service tank with the standby tank and immediately starts regenerating the tank that was just taken off line - out of service. Which means you normally are still using water during at least part of the regeneration so you get to share your water flow with the flow out the softener's drain line.
A normal two tank softener has a 24 hour reserve programmed into the K of capacity but not put on the meter so when the meter runs down to zero gallons remaining, you get to run on the reserve capacity until the next 2 AM when the regeneration starts. Done right you will rarely to never use up all the reserve capacity before the unit regenerates at 2 AM. All twin tank softeners use softened water (capacity=salt) to regenerate with softened water.
The only time a household needs a twin tank type softener is with shift workers etc. that are up'n about consistently using water at night from 2AM- 3:15 or 3:30 AM when a regular softener would be regenerating about once every 7-9 nights.
You have to program a time of day that regeneration is to start. Usually that is 2 AM and factory default on all regular two tank control valves that I've seen over like 24 years.
So how do you get one to regenerate immediately whenever the gallons count down to zero gallons remaining?
There is no reserve when programing the SXT for the immediate regen, if the unit can treat say 1000 gallons then the meter would be set for 1000 gallons.
When the 1000 gallons is used then the unit with a no hardwater bypass piston goes into a cleaning cycle or regen then no untreated water will go into the holding tank while the unit is regen, when the unit gets to the rapid rinse and brine refill treated water will then be able to go into the holding tank.
I know how metered immediate regeneration works. I don't know why you would mention it or use it in this case but...
How do you get water to it so it can regenerate when you say; "when water is going from well to softener to holding tank,"?