I have one 300W Hydor external heater on my 220G glass tank. It's located in my 68F basement and it keeps the tank at 78F with no problem. I also have a lot of rock in my tank.
You need to consider the average room temp and what the fish required temp is.
never said dont use 2 heaters but i only ever set one heater at 82 degrees the other is set at 78 just incase. But you dont need 2 heaters to maintain heat. Also water temp will only fall to room temp so unless you live in a ice box your tank shouldn't drop below what ever room temp is in my case 75+ degrees.