Separating a mixture of two types of food also constitutes a violation of Borer. Even if both foods are edible, since they are different types and one is interested in having each type separately, each is considered psolet in relation to the other. By separating them, one thereby improves them and thus transgresses the prohibition of Borer (SA 319:3; BHL s.v. “le’ekhol”).
Thus, if almonds and walnuts are mixed together, but one only wants the walnuts, the almonds are considered psolet for him. He may remove the walnuts from the mixture to eat immediately, because that is derekh akhila. But if he takes out the almonds, this is considered derekh melakha and he transgresses a Torah prohibition. If he wishes to serve guests almonds and walnuts separately, then they are both considered okhel, and he may separate them from each other to serve immediately. However, one may not separate them and then only serve them later (BHL 319:3 s.v. “hayu lefanav”).
If two foods taste different, they are considered discrete types. Therefore, if roasted meat is mixed with cooked meat, or pieces of chicken are mixed with pieces of turkey, they are treated as two different types of foods, and one may not separate them. In contrast, when all the pieces of meat are of the same type but merely different sizes, there is no prohibition on separating the big pieces from the little ones (Rema 319:3 based on Terumat Ha-deshen).