If a player breeds two trader llamas together this does not count towards the "Two by Two" advancement which requires the player to tame all of the mobs in the game. I believe it is intended that breeding the trader llamas together should count as breeding llamas (which is one of the types of mobs you must breed). However, if you classify trader llamas as a different mob then normal llamas you should have to breed trader llamas for the advancement as well.