Final answer:
Dark and white chocolates are generally not mixed due to different melting points and compositional factors that can affect the final product's texture and taste.
Step-by-step explanation:
The question about when not to mix dark chocolate and white chocolate does not pertain to a specific time, but rather to the technique and purpose within the context of cooking or baking. Generally, dark chocolate and white chocolate are not mixed because they have different melting points and compositions which could affect the texture and taste of the final product. However, there might be specific recipes that call for a marbling effect where both chocolates are used but not completely blended.