Gemara

(Mishnah): If a man married a woman and she died, then he married a second woman and he died, the second woman or her heirs collect before the heirs of the first wife.

90b: Inference #1: Kesuvas Benin Dichrin (the enactment that sons inherit their mother's Kesuvah) applies even when the husband dies in the middle (one wife dies before him and the other after him), and we are not concerned for quarrels.

We learn this from the wording of the Mishnah. It says that the second wife and her heirs come before the heirs of the first wife. This implies that if there is enough property, the heirs of the first wife also receive.

Inference #2: The Mishnah teaches that collection of a Kesuvah from the estate is considered like having an extra Dinar above the Kesuvos, and the enactment applies (whenever the husband dies in the middle. Kesuvas Benin Dichrin does not apply when it would totally uproot inheritance (52b), but here it is as if the heirs inherited and the latter Kesuvah was collected from them.)

We learn this from the fact that the Mishnah does not say that there must be an extra Dinar.

Objection #1 (Rav Ashi): The Mishnah does not prove the first law!

Perhaps Kesuvas Benin Dichrin does not apply when the husband dies in the middle. Rather, the Mishnah teaches that the heirs of the second wife inherit before heirs of the first wife.

Objection #2 (Rav Ashi): The Mishnah does not prove the second law!

Perhaps a Kesuvah is not like an extra Dinar. The case is, there is an extra Dinar in the estate!

91a (Mar Zutra): The Halachah is, Kesuvas Benin Dichrin applies when the husband dies in the middle, and a Kesuvah is considered like an extra Dinar.

Rishonim

Rif and Rosh (49b and 10:2): The Halachah is, Kesuvas Benin Dichrin applies when the husband dies in the middle. The Halachah is, a Kesuvah is considered like an extra Dinar. However, if both wives died in his lifetime, Kesuvas Benin Dichrin applies only if there is a Dinar more than both Kesuvos.

Ran (DH Kesuvah): Some say that one Kesuvah (of a wife who outlived her husband) is considered extra to enable Kesuvas Benin Dichrin only when there is enough to pay both Kesuvos. The Rashba rejected this.

Rambam (Hilchos Ishus 19:7): Yakov married Leah and Rachel, and Leah died in his lifetime and Rachel died after he died. He had sons from both of them. Even if he did not leave more property than both Kesuvos, if Rachel took the widow's oath (that she did not receive her Kesuvah yet) before she died, her children inherit her Kesuvah first, for they do not inherit through Kesuvas Benin Dichrin. Rather, it is Torah inheritance. Afterwards, Leah's children inherit her Kesuvah. If anything remains, it is divided equally. If Rachel died before swearing, Leah's sons inherit her Kesuvah, and they split the rest equally.

Rambam (8): If both wives died after Yakov, if they swore, each woman's sons inherit her Kesuvah mid'Oraisa, not through this enactment. Therefore it does not matter whether or not there is extra. The sons of the first (Leah) collect before the sons of the second. If the women did not swear, the sons split everything equally. No Kesuvah is inherited, for a widow does not inherit without swearing.

Rosh (10:2): R. Chananel says that when the husband dies in the middle, we find that Kesuvas Benin Dichrin applies. However, we do not find that it applies in the life of the second wife. Some Ge'onim say so, for certainly we are concerned for the strife this would cause, for the latter's children receive only (Bach - nothing, not even) their mother's Kesuvah, and the former's receive everything. Some Ge'onim say that even in this case Kesuvas Benin Dichrin applies. They learn from the Mishnah, which says that the second wife and her heirs come before the heirs of the first wife. The Gemara inferred that if there is enough property, the heirs of the first wife also receive. This inference applies also when the second wife is alive. Also, if we would say that in the lifetime of the second, the heirs of the first do not receive, we would say that the latter's children already merited the property. They would keep it even after their mother dies.

Mordechai (235): A Gaon was asked whether we collect when one of the wives is alive. He answered that it is not explicit in the Gemara. We do not judge such a matter; we never heard of collecting Kesuvas Benin Dichrin in such a case. Others say that we collect. Rabbeinu Meir says that since Rabanan argue, we are passive and do not collect, especially since some Ge'onim say that we never collect Kesuvas Benin Dichrin nowadays.

Poskim

Shulchan Aruch (EH 111:8): Yakov married Leah and Rachel, and Leah died in his lifetime and Rachel died after he died. He had sons from both of them. Even if he did not leave more property than both Kesuvos, if Rachel took the widow's oath before she died, her children inherit her Kesuvah first, for they do not inherit through Kesuvas Benin Dichrin. Rather, it is Torah inheritance. Afterwards, Leah's children inherit her Kesuvah. If anything remains, it is divided equally.

Chelkas Mechokek (8): The same applies even if there is not enough for both

Shulchan Aruch (ibid.): If Rachel died before swearing, Leah's sons inherit her Kesuvah, and they split the rest equally.

Chelkas Mechokek (10): In this case we are concerned lest Rachel collected her Kesuvah, therefore her sons have no claim to it.

Beis Shmuel (9): Perhaps the enactment applies even if he left only enough for Leah's Kesuvah, for we say that Rachel already collected hers. However, since Kesuvas Benin Dichrin is only an enactment and we are unsure whether or not Rachel took anything, perhaps they split all the property equally.

Shulchan Aruch (ibid.): All the more so if both wives died after Yakov, each woman's sons inherit her Kesuvah even if there is no extra, for it is a debt. The same applies if he divorced one and the other died in his lifetime, the children of the widow receive her Kesuvah even if there is not an extra Dinar. The Kesuvah of the divorcee is a debt. It is considered 'extra' over the other Kesuvah. Therefore, the sons of the divorcee collect first, and then the sons of the widow.

Beis Yosef (DH v'Ha): When one wife died in his lifetime and one died after him, the latter Kesuvah is collected like a debt. This is the ultimate inheritance! The property fell to the heirs, and they do the Mitzvah of paying a debt. Therefore, we say that there is an extra Dinar, and the heirs of the first collect Kesuvas Benin Dichrin.

Shulchan Aruch (10): If one of the wives died in his lifetime and the other is alive, this is like when one died in his lifetime and the other died after he did.

Beis Yosef (DH ve'Ka'asher): We follow the Rosh, Ran, Magid Mishneh, Ramban and Rashba against the Mordechai in the name of Rabbeinu Meir.