When do I get 2B's and A2's outfits?
Solution 1:
To get 2B and A2's outfits, you need to do more in their respective Colosseums than you had to for 9S's costume, which only required you to beat the first trial.
To get the Revealing Outfit for 2B, you need to win the rank S challenge in the Flooded City Arena. This is the sixth rank of challenges in this arena.
Similarly, for A2's alternate costume, you need to beat the 6th trial in the Desert Arena. It's worth mentioning that you don't have to do this with A2, as I beat the first six trials in this arena as 2B.