Nice job on the butt Lemans,
FWIW, cutting an 18 lb butt in half won't cut the cook time in half, unless you cut it horizontally through the center and reduce the overall thickness of the butt. Say the butt is 14 inches long, 12 inches wide and 5 inches tall. IF you cut it side to side, you'll have 2 hunks of meat that are 7 inches long, 12 inches wide but still 5 inches thick. If you cut it front to back down the center, both pieces will be 12 inches long, 6 inches wide and still 5 inches thick. The smallest dimension is what determines the cook time. In this case, it's the 5 inch thickness of the butt.
If you were to cut it horizontally, each piece would be 14 inches long, 12 inches wide but only 2.5 inches thick, which would reduce the cooking time.