It appears you can't have both a right-hand pistol and an assault rifle in your costume at the same time. You'll always either have the pistol missing, or the rifle missing. If you set the missing item to be present, the other one will disappear. When would you need both those weapons on the same character? My Dual Pistols/Martial Combat blaster has Munitions Mastery, from which she's taken the LRM Rocket power, which fires out of an Assault Rifle model. There may other ways to get into this situation, this is the only one I've tried. I've attached some screenshots demonstrating the issue inside the costume editor, I didn't want to spend the inf to prove it, but this situation persists in the game as well.