Electric water heaters never power BOTH the upper and lower elements at the same time.
If the water in the tank is cold, either the top OR bottom element will be powered through the upper thermostat (t-stat), never both at the same time. The upper t-stat will send power to the upper element until the temperature of the water in the top of the tank reaches the set point of the t-stat say 140 degrees. When the water temperature in the top of the tank reaches 140 degrees, the t-stat is "satisfied" and disconnects the power from the top element and sends it to the lower t-stat.
IF the temperature of the water in the lower half of the tank is lower than the set point of the lower t-stat, the t-stat closes to send power to the lower element. The lower element REMAINS powered by the t-stat until EITHER the water in the lower part of the tank reaches the set point temperature of the lower t-stat again, say 140 degrees; OR the temperature of the water in the upper part of the tank drops, at which point the upper t-stat "calls" and switches the power away from the lower t-stat and sends it to the upper heating element instead. When the top t-stat is satisfied, it switches power back to the lower t-stat. The process will continue back and forth for as long as needed to bring the temperature of the water in both halves of the tank to 140 degrees.
You must wait until the top of the tank is heated - before the lower element will be powered to heat the water.
I hope this helps you understand how the switching happens in these water heaters. Good luck! Please rate my reply. Thank you.
If you have a multi-meter, you can read the resistance of the element to determine if it is good. Set your meter to Ohms and to the lowest multiplier or "scale" ("Rx1" is good - some meters do not offer a multiplier however). With power to the water heater OFF, remove one wire from the heating element terminal - it does not matter which one. Insert the probe lead plugs into the "Common" and "Ohms" (or "Resistance") jacks. Touch & hold the test probes against each other and read the meter. You should see "0" or a very low value displayed. 
"Zeroing" the meter. 0.2 Ohms is displayed
Some meters have a "Zero Adjust" knob - you should rotate this knob while holding the probes together so that the display shows 0 ohms. Do not worry if yours doesn't have this feature. Next, touch one probe to a terminal of the heating element. Press the other probe to the other terminal. If your water heater is rated 4500 watts and 240 volts (this the most common rating), you should see about 12 ohms resistance on the meter display. 
A "good" element under test. 12.5 Ohms resistance displayed.
This tells you the element will heat when power is applied. If you see a value that is more than 20% off of this value, then the heater is suspect. When the elements go bad - the resistance values jump to the thousands (and much, much more) of ohms. Some digital meters show a value that is too high to display correctly as "OL" for over load, like a digital calculator.
If you have further questions - please ask! I'd be happy to help some more.
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