Hold off on the livestock.
You need to remove the phrase "it is doing fine" from your vocabulary. It indicates that all is well, yet your other posts clearly state there is a problem.
All the water in the entire system should be the same temperature at all times. Are you using different thermometers to determine that there is a problem between upstairs and downstairs?! If yes, put both in the upstairs tank and measure the water for 5 minutes. Then take them both downstairs and measure the water of the sump for 5 minutes. Then take both and measure the temperature of the water in your refugium.
They should all match.
Your heaters should be coming on at the same time and turning off at the same time.
You did not state the size of the downstairs heater. How many watts is it? 300w should be enough for your entire setup, and it doesn't matter what the number on the heater states. What matters is the water temperature of the system.
Most of my heaters are set around 76 - 77F, but they heat the water to 79-80F. The reason they do that is because the numbers on the heater rarely match reality, in my experience. Once the tank temperature is where I want it, I turn the heater off so the light clicks off, then turn it up slightly to make the light flick back on, then twist it down again enough to shut off the light (and heater). That is the set-point, and it doesn't matter what it says on the heater.