X3 - I never add wood after a smoke begins. I will start out with 4-6oz of wood depending on the length of time I anticipate the meat will take, and I don't touch the smoke box again until the meat is finished, and I start the cleanup. The heaviest smoke will appear in the beginning, and it gets lighter as the smoke time continues. So don't worry if you are not seeing heavy smoke for the duration.
The original question here was about doing consecutive smokes -- I have not had an experience of two smoke sessions in one day, but I would add new wood before the second smoke begins as described above.