I'd put the split just as the Company gets its first view of the Lonely Mountain. That'd come right after the
Barrels Out of Bond sequence (which I hear is being reworked to be more like an action-packed prison break). Also coinciding with that sequence is the confrontation between the White Council and the Necromancer. Both sequences (the prison break and the confrontation) would make for a natural climax to
An Unexpected Journey.