I’m trying to decide if this is a trick question..?
My instant thought would be set them both to 78 and they will stop heating when they sense the temperature is that level?
I’m going to watch this for other answers
I would have gone for 2 x 200watt heaters and set them both to the same temperature. Have one heater at each end of the tank.
Fully submersible aquarium heaters should be placed near the bottom of the tank and put on an angle so the heating coil is slightly lower than the thermostat. The heating coil is at the bottom of the heater and the thermostat is normally at the top of the heater attached to the cord.
Heater set points are merely reference guides and are typically not accurate at all. You need to use a thermometer to ensure an accurate setting. I like to place a new heater in a 5g bucket of cool tap water and starting with the set point indicator on the rheostat, I stir the water occasionally to ensure against thermal gradients and adjust the heater as needed to achieve the desired temperature using a thermometer. These days I use one of those fancy thermometers jobs with the lazer beam, but a regular old thermometer works just fine. After you have the desired temperature, allow the heater to cool some, then it can be placed in any tank w/o further adjustment.
The 'rule book' says you need a 275w heater to heat a 340L (90g) tank 5° above room temperature and 450w to heat the tank 10° above room temperature. So odds are @Colin_T is correct that two 150w or 200w heaters would typically be fine. Frankly, the 300w heater alone would do the job, but two lower wattage heaters can have an advantage if/when one heater sticks on. Now I've never had a heater stick on (but some have) as when my heaters have failed, they just stop working!
EDIT: Having written the above, I have some tanks where the heater wattage far exceeds recommended levels. For example, my 60g display tank has a 500w titanium heater with a Finnex external controller. My 110g Stock tank in my unheated basement has an 800w titanium heater with a Finnex external controller. Yes if/when a heater sticks on without a doubt, it can spell disaster. On the other hand, a higher wattage heater doesn't have to work as hard to maintain a set temperature...and using two properly set heaters in a tank, means that each heater works half the time that otherwise one heater would work.