Yevamos, Chapter Ten, Mishnah 7. This Mishnah deals with the same case discussed earlier, except that the previous Mishnah spoke of a nine year old boy who preceded his brother, while here the Mishnah deals with two nine year old boys: a nine year old who was intimate with his yevamah, and afterward his brother, who is also nine years old, was intimate with her.
The view of the Tanna Kamma:
According to the Tanna Kamma, the second one disqualifies her for the first. When an adult performs yibum, his yibum is complete and total, and nothing that happens afterward affects it at all. But the intimacy of a nine year old is only like a maamar: it is not a full acquisition and it does not acquire her completely. Therefore there is room for the intimacy of the second nine year old to take effect, and consequently it disqualifies her for the first.
The view of Rabbi Shimon:
Rabbi Shimon disagrees and holds that the intimacy of the second one does not disqualify her. The root of his disagreement lies in how the intimacy of a nine year old is defined: the Tanna Kamma sees it as a partial acquisition, like a maamar, which acquires part of her and leaves part of her outside his acquisition; Rabbi Shimon, however, sees it as a doubt - it is uncertain whether it is effective or not, meaning either it is fully effective or it is not effective at all.
Therefore, in his view, in either case the intimacy of the second one accomplishes nothing:
If the intimacy of a nine year old is a full acquisition: the intimacy of the first nine year old took effect completely and the yibum was finalized through it. And once the yibum has been finalized, nothing that comes afterward has any effect on her, and the second nine year old is irrelevant.
If the intimacy of a nine year old does not take effect at all: neither the intimacy of the first took effect nor the intimacy of the second, and the act of the second cannot change anything.
From this Rabbi Shimon concludes that the intimacy of the second one does not disqualify her for the first, since it has no effect whatsoever, neither one way nor the other.
In summary: in this Mishnah we dealt with two nine year old boys who were intimate with their yevamah one after the other. According to the Tanna Kamma, since the intimacy of a nine year old is only like a maamar and is not a full acquisition, the intimacy of the second disqualifies her for the first. According to Rabbi Shimon, the intimacy of a nine year old is a doubt - either it is fully effective or it is not effective at all - and therefore the intimacy of the second does not disqualify her, since on either side it cannot accomplish anything.