User blog comment:B214/Volume 23 discussion blog/@comment-30924344-20170319035400/@comment-10330822-20170319050020

That's the thing, we don't know. Until the forms actually surface, I doubt will find out. All we know is that they've been foreshadowed since Volume 21.

To tell you the truth, I didn't even realize the BxB and CxC connection until they mentioned it in Volume 21. I was like "this man is a genius, I didn't even notice that".

This volume was more talking along the lines of even though AxA would logically be below BxB, Ajuka seems to think it would be "better" than DxD, implying that this, what very well might be a natural ability via GR, "AxA", is a game breaker. It's also going to be the next thing Issei gets, as Azazel was hinting that ExE comes after the ExE group is formed, much like how Issei unlocked DxD when he was in the DxD group.