For the benefit of those who do not have your experience the problem is:
If one defendant (the other guy lets say) made a statement implicating AA and himself, the prosecutor will want to use that statement against bad guy #1 at trial. However, the jury for AA can NOT be allowed to hear that statement, because AA has a Constitutional Right to challenge that statement, BG #1 has a right to remain silent at trial and can NOT be called as a witness except by himself, and you can not cross examine a piece of paper.
So normally, if a prosecutor wants to use statements against the defendant who made them, and they reference the OTHER defendant, you need two trials, or two separate juries, and at appropriate times one jury is excluded from the proceedings.
Since it is stated that the Defense Attorney is asking for a severance. my guess is that he sees a conflict of interest where the lawyer for the co-defendant will ask questions/present evidence in such a way that it will prejudice AA.
But he may not get the severance, because that is the reason that each accused gets a separate lawyer.
Of course, this is all speculation. NOT legal advice. Just speculation.