The Gemora states: Although the Mishna had stated that if someone makes two consecutive oaths that he will not eat something and he eats it, he has transgressed one prohibition (for the second one does not take effect), if the person asked for the first oath to be permitted (and it was), the second oath is now valid.

There is a discussion if the second oath (before the first one was annulled) is regarded as an oath taken in vain (similar to an oath to nullify a mitzvah).

The Ritva writes: If he has the first one annulled, the second one is not regarded as an oath taken in vain, for now, it takes effect. He, therefore, would not incur lashes for the second oath.

The Nimukei Yosef maintains that even while the first oath is intact, the second oath is not regarded as an oath taken in vain. This is because it has potential for being a valid oath, i.e. if the first one is annulled. Others say that it is not considered in vain for the second oath is essentially an oath in order to uphold a mitzvah (fulfilling the first one), and we learned above that this is an admirable thing to do.

Rashi in Shavuos explains the reason why the second oath takes effect after the first one is annulled. He says it is because a sage has the power to retroactively revoke the oath in a manner as if the oath was never uttered. Accordingly, the second one is valid, for there is no first one any longer.

The Shach writes that according to this, the second one will only take effect if a sage annulled the oath. However, if the husband annulled the oath of his wife, her second oath would not take effect. This is because a husband does not retroactively revoke her oath; it is only considered void from this moment and on. The first oath is not regarded as if it never existed and therefore, the second one does not take effect.

Reb Akiva Eiger disagrees. He states: The reason why the second oath does not take effect initially is because one prohibition cannot take effect upon another. The second prohibition exists, but it is in a pending state. As soon as the first prohibition is removed, the second one “wakes up” and takes effect. This would be true when the husband annuls the oath of his wife as well.