We have redundant systems on just about everything important. For example, when our water pump decides to take an unexpected vacation, we can turn a couple of valves and have the other water pump step up to the plate. We like to have one operated by DC (battery) current and the other by AC current. Our third source, of course, is just the dockside city water hookup. When we are out of power, we can still run off of battery power (or just start the generator!).
If you set up your water heaters with manual shut offs much like we do with water pump systems, where you control what's being used (that's the key - "manual") by bringing one online and the other offline, you make the system use whichever water heater you direct it to use. It doesn't take long to heat a tank of water when you find out your main water heater went ka-pooey. We do this kind of plumbing with lots of things - water, sewage, fuel systems, electric, air conditioning, etc.
As for automation....you know as I sit here and think about it...this may possible with a little creativity. You could use electrically controlled solenoid valves controlled by a relay and a sensor set up to know when a water heater failed (could be a temparature sensor that triggers a relay which triggers the solenoid which moves a valve) and would then move the valves into the "other" position taking the failed heater offline and bringing the auxillary heater online. When a sensor tells a relay to "swap", an electrically controlled magnet moves a valve and allows the pipe contents to start running in a different direction. Another relay could control the on/off of each water heater.
It works for me with refrigerant gases (air conditioning), and I bet there are Alco or Sporlan valves and electrical coils that would control water flow, as well. As you can see, maintaining a boat has taught me to think outside the box. LOL If avoiding the failed water heater is important and you want to have some automation to the fix, you may want to looking into the solenoid and coil idea, but don't expect your local plumber to come up with this. It's likely a system you'll have to research and engineer yourself. But, hey, if I figure this out and understand it, it's not that hard.
The water heater was the first major appliance we had to replace and what a nightmare. It didn't fit out any of the doorways - had to beat it to death with a sledge hammer to crumple it enough to get it out the door and getting the new one in was no picnic. It lasted 26 years, though.
They don't make stuff like they used to. The ice maker lasted 27 years, and the washer/dryer lasted 28 years. I don't even want to think about what might be
next..