Answer #2: In Beraisa #2, he made it Hefker in front of three people. R. Yosi considers this to be proper Hefker (and he cannot retract). In Beraisa #1, Hefker was done in front of two (or less). This is not Hefker mid'Oraisa (he can retract until someone takes it).

(R. Yochanan): Hefker in front of three is Hefker. In front of two it is not Hefker.

(R. Yehoshua ben Levi): Mid'Oraisa, even in front of one person it is Hefker;

Chachamim said that it should be done in front of three, in order that one of them could acquire it and the other two could testify to this.

(Mishnah): If each of two partners (in a courtyard) vowed not to benefit from the other, neither may enter the courtyard;

R. Eliezer ben Yakov permits them, since each enters his own portion.