Hello: This shouldn’t require divine intervention, but you may need to sit with the heater for a bit. One test is to make sure power is switched to the lower thermostat when the upper one is satisfied. You should hear it click and be able to measure high voltage at the appropriate element as the upper thermostat is turned up or down. Likewise, when the lower thermostat is powered, you should be able to turn high voltage to the lower element on and off by turning the thermostat down or up.
Does any of that make sense?