Can I mix flat paint and semi-gloss paint?

Flat paint is the traditional type paint used on my walls in the typical American home. Just as its name states, it is flat in appearance, having no gloss or sheen. This type of paint isn't as durable as semi gloss and gloss paint, and isn't as washable. Semi-gloss paint is generally used in hallways, bathrooms and kitchens, and on woodwork. It is very durable and washable. It has a low gloss finish. Since these 2 types of paint are so vastly different, they shouldn't be mixed together.