Tosefta על תרומות 7:18

We may not mix together pure terumah [and] doubtful [terumah] (i.e., terumah as to which there is a doubt as to its purity, e.g., if it is uncertain which of two piles is pure and which is impure, see Lieberman) with each other, but we may mix together terumah that became impure through a secondary source of impurity with terumah that became impure through a primary source of impurity, even though impurity is added onto impurity. If he said, "This doubtful [terumah] is pure," behold, it is pure! If he said, "It is incumbent on me to wait until I find out about [its status]," behold it is impure! [If] a doubt arose concerning impurity in a loaf of bread next to him, [or] a doubt concerning the impurity [of a loaf of bread] on his table, he takes it and secretes it in a hidden place.