פירוש על קידושין 92:8

The first answer is that when there are two pots or other type of container, the person will understand that the unperforated pot still needs terumah to be separated. The other pot cannot be used. But if we say that the priest gets to keep the hallah separated from flour, people will not think that more hallah needs to be separated from the dough before it can be eaten. This is a case of one container.