Why does my bearded dragon hiss at me?

We just got this bearded dragon about 3 weeks ago(Luigi). I handle him daily and he has been really sweet. Well 2 days ago, we added another bearded dragon into the tank(Mario). Luigi had a big problem with Mario the frist day, but got over it the next. We rearranged the tank because Luigi was being very dominant. They are both still very little, but when I go to hold Luigi now, he puffs his tummy and hisses. I still continue to pick him up because I dont want him becoming aggressive and thinking he is the boss. They are still too young to determine sex. Like I said, Mario and Luigi and okay with eachother now, but Luigi is being quite aggressive. Will his aggression continue or will he get better?? (i refer to the dragon as a "him" I am not sure if it is a male or not). Any advice would be helpful. Thank you.