Halving a brisket won't cut the smoking time in half, but it might decrease it somewhat. Thickness of each half will affect the smoking time, as will which half is being smoked, the point side or the flat side.
The most important thing is to monitor the internal temperature as it smokes. Wait for the temp to reach 180-185 for sliced, and 190-200 for fall apart tender. Smaller pieces of brisket need to be basted regularly to prevent drying, especially if they don't have a good layer of fat covering one side.
