As perfectly_flawed said, if they are housed together you will have issues.
Otherwise, a dragon under 18 months of age cannot be overfed. What sarge said about feeding insects is right, but that's about it. You can find an excellent nutrition list here.
http://www.beautifuldragons.503xtreme.com/Nutrition.htmlDon't waste your time and money with books, many of them contain outdated and even harmful information.