Just noticed that the house has only one circuit for the two 220 hot water heaters. I am not sure how the breaker has not tripped in 3 years. The heaters are plumbed in series, but would think at some point both heaters would be on and trip the breaker.

What gauge wire and what amp breaker was used. They are plumbed in series but are the wires two runs from the panel or is it in series too?

30 amp two pole breaker. 3 wire. 10 or 12 not sure on gauge. Circuit come from main to sub panel with pull type cut off. Wires are parallel from sub panel to each heater.

and the kW rating of each heater is?
add them together and if the total is less than 7,200 watts then a 30 ampere
breaker may never trip....

and the kW rating of each heater is?
add them together and if the total is less than 7,200 watts then a 30 ampere
breaker may never trip....

4500 w each total of 9kw

how many total gallons of hot water in both heaters? How many people in
the dwelling who use hot water?

It's likely with two heaters plumbed in series that you have (4) heater elements controlled by (4) associated thermostats. The only time all (4) elements will be turned on is when you have exhausted a large part of the stored hot water and then only if the temperature settings of all the (4) thermostats are not satisfied.

What are the thermostat settings? This may give you the answer...

how many total gallons of hot water in both heaters? How many people in
the dwelling who use hot water?

It's likely with two heaters plumbed in series that you have (4) heater elements controlled by (4) associated thermostats. The only time all (4) elements will be turned on is when you have exhausted a large part of the stored hot water and then only if the temperature settings of all the (4) thermostats are not satisfied.

What are the thermostat settings? This may give you the answer...
In typical water heater installations and factory wiring, both elements NEVER come on. The top element thermostat is double pole double throw and it prevents that from happening. It has nothing to do with exhausting the hot water.

Ding!
you learn something new everyday...
It's called non-simultaneous operation and most residential water heaters come this way from the factory...this should be noted on the label. So the max load on the circuit is 4500/240= less than 30 amperes....