There is absolutely no reason why you'd have to train a bodypart twice a week to make it grow. Muscle growth is caused by an increase in the sarcomeres within the muscle fiber, as well as by an increase in the number and thickness of the myofilaments within each sarcomere. These adaptations to weight training occur when the muscle is forced to respond to a resistance that it has not experienced before. That is the entire theoretical basis for progressive-resistance exercise. Muscles grow because of an increase in your training intensity. Intensity is the key variable for muscle growth-not volume and not frequency.
The twice-a-week theory was developed because of steroid use. One of the short-term benefits of steroids is that they enhance recuperation by inhibiting cortisol production, which short-circuits the catabolic process. With their recuperative powers thus improved, lifters who use these drugs are often able to train their bodyparts twice a week and still grow. Natural bodybuilders need to throw out this concept.
It's a fact of life that natural muscle growth is a slower (yet more permanent) process. A twice-a-week-per-bodypart workout regimen will result in overtraining for all but the most genetically gifted. For the rest of us the exercise cycle will simply have to take longer.
Your frequency of training should be dictated by your recuperative abilities, not the day of the week. Your body doesn't know whether it's Tuesday or Friday. The body functions on much longer cycles and biorhythms, and drug-free athletes need to respect these natural fluctuations and work with them. You should never train a bodypart unless it has been at least a day since it felt sore. In time you will actually be able to flex a muscle and determine whether it is sufficiently recuperated and recharged to allow you to achieve progressive resistance when you go back to the gym. If it doesn't feel that way, take another day off. That recharged feeling will come with the added rest, and then-with full vigor and animal intensity-you'll blast your way to muscle growth. If this system means that you only train a bodypart three times in two weeks or even once a week, so be it.
Scientific research has shown that in healthy athletes the detraining process, where your muscles start to lose their strength through lack of use, doesn't start for two weeks, with significant losses taking up to a month. So don't worry about losing muscle if you don't train twice a week. In fact, for a committed natural athlete the chances are greater of losing muscle through over-training if you do train twice a week. Train with total intensity, but train intelligently and in harmony with your body's abilities. Your muscle gains will be much greater in the long run.