פירוש על קידושין 101:3

Abaye now cites the example of cattle tithes. The cattle tithe is performed by letting animals out of a pen, counting them, and every tenth becoming the tithe. If two come out at the same time and the owner calls both “tenth” then one is the tithe and one is not but we do not know which is which. But animals cannot be partially tithed. Yet nevertheless, although one cannot call both the tenth and the eleventh animal tithe consecutively, when one calls them both tenth at the same time, the statement takes effect.