"Twice-per-day training is the fastest way to gain strength and size, if you can afford the time. Typically, you'd do heavy, or "neural" training in the morning, and more time-under-tension training in the evening.
Now, these are relative values. For example, an Olympic lifter may do doubles and singles in the morning and sets of six in the afternoon. A bodybuilder may do six reps in the morning and 20 at night. So, you have to decide what's neural for that lifter.
So, as a rule of thumb, you want to recruit higher-threshold motor units in the morning, and lower-threshold motor units in the afternoon. Or, you can do regular training in the morning and eccentric (negative-only) training at night.
In most cases, the same body part should be trained in both sessions — heavy in the A.M., lighter in the P.M. One example would be 4 to 6 reps in the morning, 12 to 15 reps in the afternoon.
If strength is your main concern, you'd 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. So, powerlifters will do back squats twice per day; bodybuilders would do bench presses in the morning and incline dumbbell presses in the afternoon.
Also, you must leave four to six hours between workouts. This time spread is critical. If you use a shorter time spread — just two to three hours between sessions, say — you'll be too fatigued.
As for each session's length, you could start with 20 minutes in the morning and 20 more at night. From there, you'd work up to an hour each session. You have to take about 11 weeks to get to two full-hour sessions per day.
Two key points: First, for every 10 days of two-a-day training, you've got to do five days of 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 back to lifting twice per day.
Second, without a proper post-workout drink after each session, it's impossible to recover from this type of training. "
-Poliquin