With three women in a household, a coin operated water heater would be the only way to train them that hot water is not a given right. I grew up with three sisters and mom. One day my dad shut the water off while my oldest sister taking a usual too long of a shower. After that he won and my sister was more temperate with her shower time. Warning, that won't work with the wife.
About the only way to quickly know where the hot water is coming from, after about an hour of no water use so both tanks are at the set temperatures, place each of your hands on the hot water pipe at the WH and run the hot water from a fixture. The temperatures should feel about the same. If one seems to cool off faster, that tank may have a bad element. It is also possible that the calibration of one of the thermostat went to tilt and it turns off the element at a lower temperature.
Electric tanks have an upper and lower element and two thermostats. With a cold tank, the upper element is turned on by the upper thermostat. When it reaches it set temperature it switches off the top element and brings power to the lower element to heat the cold water entering from the bottom. Once it reaches temperature, the lower thermostat switches off the power. When water is drawn the lower thermostat will be switched on When the upper part of the tank gets too cold, the upper thermostat switches off the lower thermostat to power the upper thermostat. This sequence attempts to always deliver at least warm water if the demand exceeds the capacity of the WH.
When one element goes bad the results are different depending on which element went bad. If the bottom element is bad, only the top half will have hot water and the lower water warm. If it is just the top element is bad, the bottom element will heat the entire tank to temperature as long as no water is drawn. But as water is drawn and the upper thermostat switches on the upper element to heat the water, it cannot heat the water with a bad element and the lower thermostat will never turn on. Therefore, you can have one tank not providing heated water and thus cooling down the hot water from th eother tank.