When it comes to hypertrophy training, the general belief is that you need to give the muscle enough time between workouts to rebuild. Many say you can't train the same muscle within a 48-hour period. But I do it... and it works.
I started doing this 6-7 years ago out of necessity. I had a pretty hectic schedule with clients, and simply didn't have enough time to complete a full workout. But what I did have was two small opportunities with an open window in the morning and then another one in the evening. This allowed me two 45-minute sessions a day.
The effects were noticeable. What I found was that by only training for 30-45 minutes it allowed each workout to be focused and intense. The break between them gave me the chance to recharge and refocus prior to the second workout.
My beliefs about this method were reinforced when Charles Poliquin advocated training the same body part twice in one day. By training this way, it increases the overall volume than most would complete in one session, which is huge when it comes to gaining size. But also I've found that the quality of work is superior.
- Hit the SAME muscle group in both the morning and evening workout.
- Change the training protocol from the AM to PM session. In the morning workout, do fewer reps (6-8). In the evening session, lighten the load and do more reps (12-20). The second workout consists of more classic bodybuilding strategies: slow tempos, iso-holds, negatives, drop sets, etc. Vary the exercises from morning session to evening session.
- Recover as much as possible. Give yourself 6-8 hours between sessions and meet your nutritional requirements between workouts.
- This is not a method you should do forever. You'll need to take breaks and return to once-a-day training every 2-4 weeks.
Granted, this might not be feasible for you, but it's a method that can be a temporary 2-4 week block to overcome plateaus and stimulate more growth.