"Their tank is a 20g long, but I read a post a little down saying they need seperate housing? why?"
Because, when the females start laying eggs, they get VERY territorial. Most of the time, crocodile skinks can live together just fine, and will sometimes even group together in the same hiding spot / lair when distressed. When a female is about to lay an egg, however, she will get VERY aggressive about defending her area.
Story: I heard some odd rustling sounds from my terrarium and looked inside. There, with her head sticking out of her "hide hole" was one of my female crocodile skinks. She had another one of the skinks (who had apparently been trying to get into her hole) by the throat and on his side, two of his feet dangling in the air. I seperated them and made a third "hole" for the other skink to live in (he had previously been living under a half-log.
That was the only time I've ever seen that sort of aggression from a crocodile skink. It was very odd, considering that before that (and, in fact, a week later) they would sometimes all gather in the same hide-hole during the day.
"How do you sex them (with photos would be great)?"
You can "pop" them like a snake. A less reliable method is to look at the bottoms of their feet - males have pores on their back feet-bottoms while females have smooth feet (I was told by a breeder, however, that this method is not always accurate). I have also heard that some people just look for a scale on their stomachs (if they have it, it is a male), but I've never tried this method so I can't say whether or not it works.