Which is which?

Yesterday, AJ's therapist, who has been working with him since last July, mistook Zack for AJ and tried to bring him upstairs to begin working.  Zack backed away from her and giggled and told her "no, no, no."

I admit that if I dress them alike, I sometimes have to stare at them for a few seconds before I can discern which is which.

Can you tell?

Picture 1

Picture 2

Picture 3

Picture 4

Picture 5

Picture 6

Picture 7

Picture 8

Had to snap this one quick, before they bit each others' faces.

Put your guesses in the comments, and I'll comment with results in a few days.  Assuming I can tell which one is which.