My advice: you are in a position every candidate dreams of, so enjoy it and do not stress too much! You have a luxury of going into the BCG interview being calm and knowing you have ATK offer in your pocket (and I wouldn't call ATK tier 2, they are a top notch consulting company).
So, do the BCG interviews and you will see what happens. If they give you an offer, you will have time to think about it and decide. In case you will then think BCG would be a better fit for you, then you will just accept their offer, end of story. Consulting is a very competitve employer's market and everybody at ATK will understand. However, honestly I know people who decided for ATK against BCG, when they had both offers (because of different reasons, like industry focus of the particular office, culture, impressions from the interview, etc.), so just take your time and think about it when the moment arrives.