One thing to remember about Larry Scott's arm training is that he also did "Burns" on the Preacher and Spider moves. The bottom up "Burns" to just below a 90 degree curl, hits the Brachialis, but doesn't activate the Biceps at that point. Holding the contraction tight at that point for a good isometric 7 seconds, with a slow negative contraction down again, emphasizes the Brachialis. It isolates the Brachialis, even more than full hammer curls do, because the Biceps aren't involved. Since the Brachialis is under the Biceps, a bigger Brachialis muscle will raise the Biceps up higher, therefore creating a larger size arm, and creating the split Biceps look, too.
Scott personally discussed some of these ideas during his seminar that was taped in the 80's.
We had clips from the event posted in one of the Training board stickies.