Harriet wanted the book out for fall 2009, Brandon estimated the work from the outline/notes and thought it could be done. There was no option to split the book then, Brandon thought it wouldn't be necessary, that he could do it in one big novel as Jordan intended. He decided to work individually on each of the storylines up to TG (Merrilor), then write the last third of the book for unrelated pratical reasons.

After he finished a first throughline (Egwene's, IRRC. It's not confirmed, but Brandon once said he started with the most complex of the four and the one he thought would also be the longest) and it clocked at 150k words, and then when looking into the second storyline and estimating it might be longer than he thought originally as well, it became increasingly likely that the book would be published in two volumes for practical reasons re:binding and retail, and Brandon started hinting at that more and more. Brandon still expected however that he'd finish the whole book before any publishing decision would be made by Harriet. He had no idea what Harriet and Tor would decide to do with publishing, but he was telling the fans that anyway there was a very natural story point where volume 1 would end.

But writing was taking him longer than expected (much larger word count than expected), and he started to suspect each storyline would be roughly 150k after all, so Harriet decided to start editing his first delivered storyline to speed things up, which she never agrees to do normally (she normally insists to read a whole book to get the full picture before she edits). That was done in the hope to meet the promised target publishing date.

Then during summer/fall 2008 it became obvious Brandon wouldn't manage to reach with the four storylines the "big event that starts TG (aka how he used to refer to the Merrilor meeting before TOM came out) by the "ultimate" deadline of January 2008. He had finished two and was working on the third (material which ended up in TOM, not TGS).

Harriet and Brandon (and Tom Doherty, for advice) looked at the written material and they re estimated the work that remained to be done.

They agreed it would take too long for Brandon to finish the whole thing - that pushed the release way beyond what Harriet was comfortable with - and so they would split it in two WOT books published apart, not as AMOL vol 1&2. They were reluctantly giving up on the fall 2009 promised date, as Brandon thought he might manage to finish the 2 other storylines and the miscellaneous cluster 5 by spring or summer 2009 (that was another underestimation, btw!). Of course it would in turn take much longer to Harriet to edit that book than expected, pushing it to spring or summer 2010 for the first novel. It's in the face of that and knowing that Harriet really would have preferred to keep her promise to the fans to get the book out by fall 2009, and sharing that pressure himself, that Brandon came up with the "last resort" idea to re arrange the timelines of various events and organize what he had already written to form a first novel, focussed on Rand-Egwene, while what he'd write for Perrin-Mat and all the rest would make up a second, and the Last Battle a third. Harriet was skeptical and asked Brandon to work out the details more and come up with a finished proposal. He did, and she decided to follow his proposal and announced the book split.

Brandon has professional opinions and advice - and is expected to, that's why Harriet hired herself a Fantasy writer rather than do it herself or hire a ghostwriter - but Harriet has all the decisional power in the end. Not as the editor, but as the head of RJ's literary succession, sole owner of WOT. Brandon is the "hired help", so to speak. He works for Harriet, like Maria and Alan do.

I'm not sure if I unwillingly conveyed an appearance of conflict in there, but by all accounts there's none. Harriet and Doherty are close friends and everyone always said that RJ had a really exceptional relationship with his publisher. Doherty is involved not because Tor has much decisional power regarding WOT matters, but because Harriet (and previously RJ) highly values his opinion and advice, as she seems to value Brandon's.