If you're looking for the fastest and most effective way to build size and strength, finding the time to train twice-per-day is the best plan. To program it the right way:

  1. Do heavier or "neural" training in the morning. Do more time-under-tension, "bodybuilding-type" training in the evening. For example, do mostly sets of 6 reps in the morning and work with 15-20 reps at night. Or, you can do regular training in the morning and focus on eccentric (negative-only) training at night.
  2. The same body part or muscle group should be trained in both sessions. Hit it heavy first and hit it again lighter in the next session.
  3. If strength is your main concern, you want to do the same exercises for both sessions. If you're focused on hypertrophy, you may want to use different variations of the exercises, but again, the same body part each time. Powerlifters might do back squats twice per day. Bodybuilders could do bench press in the morning and incline dumbbell presses in the afternoon.
  4. You must leave four to six hours between workouts. This time spread is critical. If you use a shorter time spread, like just two to three hours, you won't fully recover in time and cumulative fatigue will prevent progress.
  5. Don't jump right into doing two of your normal workouts in the same day. Start with a cap of 20 minutes of training in the morning and 20 more at night. From there, gradually work up to doing more each session, building up to an hour per workout. You should take about 11 weeks to get to two full-hour sessions per day.
  6. To prevent burning out, every ten days of two-a-day training, you've got to do five days of "standard" once-per-day training to give your body a break. I recommend training for about 40 minutes in the morning on those days. After five days, you can go right back to lifting twice per day.
  7. Don't forget that more training means nutrition is even more important. Specifically, without proper workout nutrition for each session, it's impossible to recover from this type of training.

Related:  2-A-Day Training for Radical Gains

Related:  The Super-Accumulation Program