I do not agree with the advice here on heater placement. ALmost every in tank heater I have is place horizontally near the bottom of the tank. There are multiple reasons for this.
1. It is easier to hide so less ugly.
2. The heat is emitted the entire bottom half of the heater. When placed vertically it creates a narrow column of rising heated water. When placed as I describe you have several inches of heater sending up a small wall of heated water.
3. Since heat and heated water rise, it pulls other water upwards. You get more pull from a heater which is [placed horizontally bear the bottom where the coldest water in a tank naturally is. So you are heating more of the coldest water and moving even more water from the bottom to the surface.
4. Circulation helps even out the water so there are noyt so many quiet areas and things like dechlor and meds get distributed not to mention warmer water and oxygen.
The kld will help hold in the heat some.
Is there any chance that the power to the tank is off at night when nobody is there? I can heat a 10 gal. with a pair of 100w heaters placed apart in the tank even if the external temp. is under 60F. I could do it with a single 150w but I like using multiple heaters for a number of reasons. I have found that using an additional heater controller is a big help but they are not cheap.
A lot of folks will say bettas need warmish water. Depending on which site one visits, you will see the range from about 75 to 82. So, you need to insure your fish are in that range. I would start by raising the temp on the heaters. But I am wondering if they are both working correctly. The one set to 79 likely is since your tank hits that temp during the day. But if you need lore heat over night and are not getting it, I wonder about the second heater. Is there any chance it isn't working?
When you go to work tomorrow, if the temp is low put your hand into the tank near the lower half of the 50w and see if you can tell it is heating. You may as well check the bigger one, JIK.
When I do my outdoor summer tanks the overnight temps early and late in the season are the challenge. I keep fish which want water between 80 and 86 F most of the time. But during the start and end of the season I am seeing temps in the mid 50s over night. To battle that I need to use 15 wpg or more for bigger tanks (40 to 50 gal.).