I can only answer a few questions from my own experiences. I would not keep two together unless you know a few things:
1; their gender! Two males is a No-No, two females should be fine. Male and female, only if you plan on breeding and know what you are doing.
2: Were they kept together previous to your owning them? If so, they should be fine, if not, a quarantine period is recommended to make sure they are both healthy, i.e. no parasites, etc.
3: Are they similar in size weight? They should only be kept together if they are very similar in size/weight as bullying for food can happen on occassion and cause stress.
I personally use filtered water for both misting and their water bowls, as who knows whats in tap water these days. I have a Pur filter.
I feed CDG, and occassionally crickets dusted with either CGD or calcium without d3.
They should get all their needed nutrients from the CGD and that should be their main diet, IMO.
I only feed crickets every week and a half to two weeks. Mine are still juvies so they get CGD every night.
Like people, some geckos can over-eat and get chubby and some will stay naturally thin. So I guess yes, you can technically overfeed, but they are much smarter than humans in that regard, so they won't eat until they burst or anything. You'll more likely find left over food in their bowls.
I am a plant-tard/black thumb so I can't help you there, but your 20 L sounds fine for a single gecko. You will likely get differing opinions as to how many in how large a tank, so just go with what feels right to you as long as it isn't a huge geck in a 5 gal or something like that. just give it room to leap and climb and it should be fine.
Oh, I should mention this: Cresties are a lot like chips. You can start with one, but you will very likely want more! They're cute, funny, and addictive!
Welcome to the world of Rhacs!
Help! Crested Geckos! 0.1.Leachie, 2.1.1 Gargs, 1.0 Chahoua, 2.1. Blue Tongue Skinks, 1.1. White Lined Geckos, 1.3. African Fat Tails, 2.2.0 Tokay Geckos, 1.0. Giant Day Gecko, 2 tarantulas