First off, welcome to the forum Miro.
You can sex kings at any age. For hatchlings you can pop them (provded you know how, if not it is best to have someone who knows how to do so). Being yearlings you could probably have them probed to determine their sex.
Their behavior varies just like people or any other animal. The way yours are acting sounds like normal activities even know the two are acting differently. I wouldn't worry about it especially if they are eating, shedding and eliminating on a regular basis.