I have them both set for 89 (max). I have noticed they both go on at the same time when needed, both go off when not needed.
I think the electronics of it is, the higher the wattage, the faster it heats upthe water. but no matter the wattage, it will heat up the water but will take longer & stay on longer if less wattage.
I think an analogy is using your stove top or more so using your BBQ grill or outdoor stove top - the higher the BTU output, the stronger the flame the faster to cook/boil. one can test this by boiling water & seeing the difference in the time it takes to boil when using different BTU output.