it's for backup and protection... if one dies, the other can keep the tank warm. Also if one sticks in the "on" position, it's less likely to boil the tank because each heater will be smaller than one big one would be.
that said, I have 1.
what's your heater set to?
it sounds like your tank is just getting too hot in the day... 80-81 is fine, but the temp swing is not good.
54g all-glass bowfront corner planted FW
Red tiger lilly, Rotala Indica, Sagittaria Subulata, Micro Sword, Cardinal Plant
Fluorite Black sand and gravel