פירוש על קידושין 108:11

The gift given to the one separating tithes was not second tithe itself, but untithed produce. The agricultural gifts are still within the produce (mixed up in it) and are not considered as if they have been separated. Therefore, one can give, in a sense, second tithe, to another person. When he separates the tithe, it will be his own tithe he is separating.