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Hi, I'm Mike and I have 20+ years of experience as a plumber. I'll be glad to assist you today.
The first thing to do would be to check the continuity of the element with a multimeter. Almost certainly the issue is that the element is burnt out but best to check.
If you are unfamiliar with testing voltages be sure to turn the power off and you can use an analog multimeter with a setting that beeps when you have continuity. If you touch the probes together it will beep and that is the same sound you will hear when touching the probes to each leg of the element. So, turn the power off, disconnect the wires from the element then put the multimeter on the correct setting, test it by touching the probes together, then touch each probe to the each wire connector and if no beep then new element is needed.
Sounds good to me.
Are the thermostat settings on the upper and lower thermostat the same?
Is it getting power to the element and if you turn the thermostat down then you no longer get power to the element?
Sorry I had not seen this response until now. If the water heaters are at different locations that would be the cause of the issue. You have to split them up. I have a hard time explaining why but somehow when they are piped like that they draw the cold water back in from the other water heater.