A dual diet involves 2 different diets that your client will alternate between. For example, you would use a dual diet to prescribe different diets for training and non-training days, or for high carb and low carb days.
dual diets in detail
The video below explains dual diets in more detail.
Dual diets should only be used on clients with existing measurements of their body composition response to single diets.
Following a single diet, strictly, for an extended period, is the only accurate and reliable way to measure the effect of a diet. And measuring exactly how the body changes in response to a diet is the only way to learn and predict how to change the diet to cause more positive body changes. Switching between 2 diets eliminates any ability to conclude anything about either diet, because there is no way to know which diet contributed what to the result?
Recomposer tries to solve the logical dilemma of dual diets by averaging the daily nutrition in the Progress Table. But this approach is still far less certain than simply following a single diet and measuring what happens.