Having two tanks gives you the ability to have soft water 24 hours a day, 7 days a week. When your system needs to regenerate, it will begin using its other tank to provide soft water. Having a single tank system with a timer can lead to wasteful salt and water usage if your system regenerates before it truly needs to. Or, you can draw hard water into the home if your system needs to regenerate, but the timer hasn’t told it to yet.