I have my adults on crushed walnut shells , it's cheaper than sand and a lot easier to keep clean . I get it at my local reptile store . So it last longer .
<<Nooooooooo.....!!! Quite the opposite, really. Great breeding ground for bacteria and all kinds of funk. Not to mention the fact that it's not digestible and will clog up your dragon's systems if they eat any, whether intentionally or not, leading to severe impaction issues and even death... I vote NO NO NO on the crushed walnut.
I keep the male in a 40 gal tank, from head to tail he is 20 inches long.
<< Pretty standard, actually, don't think there's any Giant in there (but you never know for sure unless you bred them yourself! 
). Most of my adult males have been 18-22" long. My female is smaller and is in a 20 gal tank under his tank stand
<< 40 gallon is the absolute minimum for an adult bearded dragon, 20 is too small. 4'x2'x2' for each adult dragon is the recommended minimum home size, as kingofnobbys stated. . They are separated all of the time , I do give them bathes together and occasionally let them run around the house together , while keeping a close eye to make sure they don't mate . The babies are on a sand matt . It's safe for them to be on it and it looks like real substrate , without the harm of them ingesting anything .
