If we were going to split SoO up between the two factions, I would say it makes the most sense for the Alliance to get credit all the way up to the Gates of Orgrimmar, and then have both forces enter the city and do the Dark Shaman encounter (while both witness the barbarity of Garrosh's abuse of both Horde and Alliance prisoners in the Valley of Strength). After that, the Alliance essentially parks itself at the Gates and prevents a True Horde Kor'kron pincer movement while the Horde Insurgent force descends into the Underhold to contend with Nazgrim and the various traps, bosses, and encounters therein. With Thrall present in Garrosh's new throne room in the Underhold, as well as Saurfang before the Klaxxi Paragons, it all seems geared toward a Horde presence in the depths of the Underhold.