I disagree. With decent circulation, in a small tank like this, the temps should be pretty consistent throughout the tank. In a larger tank, that's probably true, but not really with a small one, especially two opposing HOB filters giving excellent circulation.
I would agree with that, the mixing rate with a HOB in that small of a tank is very fast. Short of having some structures that create dead spaces, or half of it sitting in the sun, I would be very surprised if the temperature varies by more than a degree or so between ends.
I also, the more I read this (and get reminded this is 20G) wonder how you can need more heat, especially for being around 80. At the moment (waiting for larger ones to come in), I'm heaving a 220 (yes, 10x) tank with a 100W heater, room temp 74, to tank temp 78. It took a while and is not the situation I plan to leave -- but unless you have big room-tank differences, it really does not take nearly as much heat as people might advise.
If your 50W heater is erratic, replace it. But with that small of tank (and I think many of us lost that in all the discussion of different tanks), I think I'd start with one 50W replacement and see how it goes before adding a second.
Note there are two other sources of temperature variation - evaporation and lights. If you have strong, especially hot lights, that can drive the temperature up more than a heater. And if you have an open top that evaporates, room humidity variations and any breezes (e.g. ceiling fans) can make a surprisingly large heat loss. Since the latter might be full time, and the former on and off, this can cause heat swings that the heater might not help with (notably getting too hot from the lights, all the heater can do is turn off).
But you are obviously cost conscious - first make sure what you have is broken. See if you can borrow an accurate thermometer; if you have some cooking enthusiasts they might have one (e.g. Thermopen's are great for this). With no other sources of heat, check it frequently with your heater, compare to the room (i.e. that it is always hotter than the room). If your heater is driving it to erratic temperatures, throw it away. If it's something else, like lights, fix that first.
If you need a new heater, I'd buy one good one first. Then test it. A LOT of heaters are going to be off a few degrees, that's easy to adjust, but what is more difficult is if it is erratic. Keep using the thermometer and make sure it is holding temperature, and adjust up or down to get what you want (e.g. you might need to set the dial to 80 to get 78).
Only if you find the heater is falling behind -- it's running, but the temperature won't go up -- do you really need a second heater.
And if you find the tank is too hot, and heater is off, look for why - lights, etc., and adjust those.
Don't take all the "5 watts per gallon" and such as needed. Or more precisely, such advice is predicated on having plenty of heat for unusual situations -- water changes where the added water needs to heat rapidly, very cold rooms, ability to heat up an extra 10 degrees if needed (e.g. to treat Ich). If those aren't concerns, a lot less heat is needed.
You need to think like a scientist, and not like a shopper.
Take it methodically and figure out what's really wrong and what you need and you will save money.