Hot water heater bypass

[Paul M.]

Looking for ways to provide short term relief for hot water tank (or combi boiler) failure so tenant has temp hot water while I seek out fix.

When a hot water heater fails it is a big problem because the tenant has no hot water heater and you are scrambling to get a plumber, who are harder and harder to get and when it is last minute you are forced to take the most expensive option and still it is a long period without hot water.   With heat, I can bring in space heaters temporarily, and in some cases I have redundant heating methods eg a minisplit that also heats but isn't primary heating method.   With cooling, I can bring in window ACs.   I try to provide class A service.

To avoid disruption, of you say have a 2 unit building, is there anything unsafe about having a set of ball valves to act as a bypass setup so that when one hot water tank fails, you could direct water from the other hot water tank to provide heat for both apartments?   Obviously if tenants pay utilities you'd have to pay tenant whose tank is providing the backup.   Otherwise if someone made a mistake with shutoffs the unintended outcomes would be 1) fixture fed by 2 water heaters, doesn't seem like issue, except water may not be hot enough and you will see that 2) broken hot water heater fed by working hot water heater (could create flooding if broken one leaks similar to regular flooding whenever a hot water heater leaks) 3) fixture fed by no hot water heaters (will be obvious issue to tenant and no harm done).     Other then #2, I don't see any risks, am I missing anything?      

Other solutions I've looked into:

have on demand hot water heater-too expensive at like 3-4k unless there is a cheaper easier one out there?

tenant boils water-well that is what I say now, trying to avoid that

a second hot water tank, say if you replace early-too expensive for tenant to operate 2 tanks at once, and you can't turn down the temp for one while it is not in use because it has to be hot to kill germs

Replace tank early-you can do this, but how long the tank will live is unpredictable could be before end of warranty or long after.   Ball valves are cheaper

Booster-seems to be just for supplementing, not clear if it can heat by itself?   Also not sure of cost.