All of us with more than 1 can probably relate....it's like children(skin), each have their own personality, some have the same characteristics, some are like night & day. I have 1 who is super cuddly, the other, not as much. Both are spoiled rotten the same!
I would just allow them to be who they are but continue to show them both the same amount of attention and maybe eventually the one that's more subdued will open up a little more...but will probably always have the recluse characteristic!