I did a bit of research and found this:
Most popular water softeners have an automatic regenerating system. The most basic type has an electric timer that flushes and recharges the system on a regular schedule. During recharging, soft water is not available. Tapworks?
A second type of control uses a computer that watches how much water is used. When enough water has passed through the mineral tank to have depleted the beads of sodium, the computer triggers regeneration. These softeners often have reserve resin capacity, so that some soft water will be available during recharging.
A third type of control uses a mechanical water meter to measure water usage and initiate recharging. The advantage of this system is that no electrical components are required and the mineral tank is only recharged when necessary. When it is equipped with two mineral tanks, softened water is always available, even when the unit is recharging. Harveys?
http://www.popularmechanics.com/home/interior-projects/how-to/a150/1275126/
I'm still not convinced that a twin tank mechanical model offers any benefits over a single tank electrical timer model for a combo boiler.
As I say, our recharge happens at 3am and takes about an hour. It's not much of an inconvenience to not have soft water from 3am to 4am.