First can two be housed together?
This depends...If you want to house 2 together you'll have to make sure they are about the same size, and both female. If you want babies then a male and female could work...but sometimes one male and one female together can really stress out the female...so you'd be best with 2 females. The only way to absolutely ensure that you get 2 females is to get adults or older juveniles that can be visually identified as females (no pores in v-shaped row of scales and no prominent bumps at the base of the tail). Also be sure to get them from the same source...or you'd have to quarantine them separately for 2-3 months.
Is a 20L tank good enough for housing.
A 20L is definitely plenty for 2 leos...you could even get a 3rd in a 20L.
do all they eat is crickets or mealworms and do you feed them on a daily basis?
Crickets and/or mealies are what most feed (daily or every other day usually), but there are other possible staples (like superworms or silkworms), and some things that can be fed as treats (waxworms and pinky mice). You also have to make sure that you supplement their food...I recommend t-rex sandfire ICB dusts (any species....they're all the same just marketed differently), it contains everything they need and you just dust their food at every feeding. If you use a different dust you need to supplement with calcium in a dish in the tank as well as dusting the food.
I use papertowels for my BDs but will eventually use sand when they get bigger is this the same with geckos?
Sand is under HUGE debate with leopard geckos...from what I've learned sand can cause impaction, however usually when it does there are other husbandry issues leading to the problem. Also, no hatchlings or juvies should be kept on sand no matter what. Also if sand is used its best to keep it to a minimum (less than 1/3 of the total floorspace). Most do use paper towels. My personal favorite substrate is tile or slate. It's more natural then anything else available alone (most natural would probably be to use slate pieces and fill in the cracks with the finest grain playsand you can find available), it looks great, its cheap (less than $2 a tile), its readily available (some carpet stores, any big home improvement store like Home Depot), its very easy to clean, it has no chance of causing health issues, and it holds heat well. Also there are a lot of choices available...ceramic, stone, vinyl, linoleum, and lots of different patterns on top of that. 12" x 12" tiles fit quite well in 20L's...with the exception of a space at the end (so about 2 1/2 tiles are needed)...to remedy that you could have a tile cut for you, or you could get vinyl or linoleum tiles and just cut one with a good pair of scissors. Oh, and you have to make sure the tiles aren't glossy so they aren't slippery.
Do they need a basking light? And do they need UVB?
Leos do not need UVB or a basking light (per se). They DO need a heat source and a temperature gradient in the tank. Generally the best choice for a heat source is an under tank heater under one side of the tank (leaving the other end cooler) attached to a rheostat or thermostat (so you can control the temp). A less expensive choice is just to use a heat lamp (not bright...a red or black bulb...not a blacklight...that can provide heat all day and night) above something that will hold heat...one of the reasons tile is nice is that it will hold the heat from the light.
Beyond all that they do need at least 3 hides...two dry ones that can be just about anything, one at the warm end of the tank and one at the cool...and a humid hide (usually a margerine tub with a hole in the side with moistened paper towels in it) on the warm side of the tank. I'd recommend you look up some caresheets (I have one on my website if you click on the logo below) just to make sure you've got everything down. Leos are FANTASTIC...and are utterly addictive...lol I started with 2 and had 5 within a month...lol Good luck!!
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Christina

Leopard Gecko Morph Descriptions
2.3 leos
-0.1 tangerine het rainwater albino w/jungle background (Blinkers)
-0.2 jungles (Vahz & Skissor)
-1.0 tremper albino (Spitfire)
-1.0 tangerine rainwater albino (Bronx)
-ignorance is not to be punished when one is trying to gain knowledge...what scares me is the vast number of people who, when given the information to gain knowledge, choose to ignore it.