A 75 watt is way to much for a 20 gallon tank and the 50 is prob alittle low. Use the same watt bulbs for day and night, a drop is not needed or desired by your snake. If sounds like the regurg is prob from bad temps.
Do you have the heat lamp resting on the screen?
I like to put my lamps on the Zoo Med light stands. They allow you to raise the fixture and therefor lower the temp of the bulbs hot spot in the tank. I would buy a pair of 60 watt bulbs and light stands if you can afford it. If not you might also buy another 50 and take the 75 watt away. If you get a tstat its hot spot will be taken care of so then you just need to keep the rest of the tank around 80. Also get regular style bulbs not basking bulbs.
My 40 gallon tank has a 75 watt ceremic heater and two 60 watt(oneday/one night) to give me good temps. My home stays at 68 and both my heat sources are raised about 4-6 inches above the tanks screen. That should give you an idea of how much heat you are pumping into your tank.