Correct me if I’m wrong but I thought electric water heaters only ran one element at a time. I don’t remember the rhyme or reason right now.
I can't speak for every water heater on the planet, but the instruction manual that came with mine said it would run the bottom as maintenance and then kick the top one on as needed to boost it. So, under load, I assume that means they both turn on when you're actually using the thing. It also called for a breaker of the size to allow it to do that.