Brandon drew a graph of AMOL's structure and explained in some details how he ended up re structuring it as three books. Not much that isn't already known in there, book 12 will have two main story lines (we know it's Rand and Egwene, but as I said Brandon didn't say so explicitly at the Q&A) and teasers for three more (Mat - and seemingly Perrin and Elayne). By 'teasers', Brandon precised he means 3 or 4 chapters per story line, the rest of the chapters being divided between the two main story lines (by recent books, this could means Egwene/Rand have about 10-12 chapters each, or a few more). Some developments happen in the teasers but it's not huge stuff, more like set ups chapters for what happens in book 13.

Book 13 will have the opposite, with 3-4 chapters each for Egwene and Rand, "toward the end". Brandon kept those for book 13 to avoid spoiling in TGS the climax of book 13, which will mark the reunion of all the main story lines at some location, and launch TG. So in book 13 we will have the residual Rand/Egwene chapters that specifically build up to the reunion.