פירוש על קידושין 101:2

One can dedicate not just whole grains, but even half grains. One can say “let half of each grain be the tithe.” So this is how we understand what happened in the case of the person who made double the amount tithe. He didn’t make 2/10 of the produce tithe—rather he made ½ of each grain of 2/10 of the produce tithe. The remainder of the produce is tithed, but the tithe itself is ruined because we can’t tell which half of each piece of grain is tithe.