It is not accurate to say that Americans, as a whole, cannot use a knife and fork at the same time. Like people in many other cultures, Americans are generally capable of using both utensils simultaneously while eating. The style of dining and the manner in which utensils are used can vary from person to person based on cultural backgrounds, personal preferences, and the type of meal being served.
In the United States, the "European" or "continental" dining style, which involves holding the fork in the left hand and the knife in the right hand throughout the meal, is commonly practiced. This style allows for cutting food with the knife and then using the fork to bring the food to the mouth. Many Americans are familiar with and use this style, especially in more formal settings.
However, some Americans may also use the "American" or "zig-zag" style, where they switch the fork to the right hand after cutting the food, and then eat with the fork in the right hand. This style is considered less formal but is still perfectly acceptable in American dining culture.
It's important to remember that dining etiquette and practices can vary widely among individuals, and generalizations about a particular group's dining habits may not accurately reflect the diversity within that group. People from different regions and cultural backgrounds may have their unique dining customs, and what may seem unusual to one person might be completely normal to another.