Heat limit switch on Sears water heater

I thought I could do this without help, but it got too complicated for me.
I have a Sears Kenmore 153.318560 52 Gal. Power Miser 8+ electric water heater.
It has two sets of heating element, thermostat, and heat limit switch. One set at the top and one at the bottom.
One thermostat is bad, I presume, but I don't know how to tell which??
It would help if I understood how it worked.** If the top heat limit switch trips, does that cut off power to both heating elements?
If the bottom trips, does that cut off power to both or only the bottom one?
**(The wiring diagram in the owner's manual doesn't show enough detail.)
For testing purposes, is there a way to trip the switch without disconnecting a wire?
On the web someone said to feel the heater to know if only the top of the heater was hot, or all of it was, but I can't feel any heat from any of it.
Both of the high limit switch have tripped during the past few days. I don't know exactly when the top did and the bottom did.
Background: Three days ago, no hot water so I turned off the breaker, opened the top cover, saw the button for the TOP heat limit switch, pushed it, and had hot water for maybe a day. After that, again only very lukewarm water, so I pushed it again. The following night, the water was up to 170 degrees!, but too tired from work to work on it. The next day, down to 150, and 6 hours later, down to 145. The only hot water I used during that time was the water to measure the water temperature, when I filled the powder room sink immediately above the water heater. I only did this twice. Is the new cold water entering enough to lower the temperature, or is it radiating heat that fast from the heater?
Eventually I think both switches tripped, and I only reset the bottom one. Now the temp is up to 180!
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