✅Answer: The origin of this question is a narration that believers act upon out of their submission to authentic Shia narrations.

✅First, please pay attention to the narration in question:

1 عَنْ حَمَّادٍ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ يَقُولُ لَا يَحِلُّ لِأَحَدٍ أَنْ يَجْمَعَ بَيْنَ ثِنْتَيْنِ مِنْ وُلْدِ فَاطِمَةَ - إِنَّ ذَلِكَ يَبْلُغُهَا فَيَشُقُّ عَلَيْهَا قُلْتُ يَبْلُغُهَا قَالَ إِي وَ اللَّهِ

📚 Ilal al-Shara'i' p. 590

Hammad said: I heard Imam al-Sadiq (peace be upon him) say: It is not permissible for anyone to combine two women from the children of Fatimah (peace be upon her) (i.e., to have two 'sayyidah' women married to him at the same time). Indeed, the news of this matter reaches Fatimah (peace be upon her) and it becomes difficult for her! I said: It reaches her?! He said: Yes, by Allah!

✅Now, the question is whether the temporary nature of one of these two 'sayyidahs' is also considered combining two 'sayyidahs' in marriage or not?

✅Please pay attention to the following narration:

2 مَرْوَانَ بْنِ دِينَارٍ قَالَ: قُلْتُ لِأَبِي إِبْرَاهِيمَ لِأَيِّ عِلَّةٍ لَا يَجُوزُ لِلرَّجُلِ أَنْ يَجْمَعَ بَيْنَ الْأُخْتَيْنِ قَالَ لِتَحْصِينِ الْإِسْلَامِ وَ فِي سَائِرِ الْأَدْيَانِ يُرَى ذَلِكَ

📚 Ilal al-Shara'i' p. 498

Marwan ibn Dinar said: I said to Imam al-Sadiq (peace be upon him): For what reason is it not permissible for a man to combine two sisters (in marriage)? The Imam said: Because of 'tahsini' and chastity, which exist in Islam, while this is seen in other religions.

✅In the Quran and numerous narrations, one example of which was mentioned, combining two sisters in marriage is forbidden and prohibited. By referring to the narrations, it can be seen that 'combining' two people is not limited to permanent marriage but includes all forms of marriage, and taking a sister-in-law in 'mut'ah' (temporary marriage) is also considered combining two sisters. For example:

3 أَحْمَدَ بْنِ مُحَمَّدِ بْنِ أَبِي نَصْرٍ عَنِ الرِّضَا قَالَ: سَأَلْتُهُ عَنْ رَجُلٍ تَكُونُ عِنْدَهُ امْرَأَةٌ يَحِلُّ أَنْ يَتَزَوَّجَ أُخْتَهَا مُتْعَةً قَالَ لَا.

📚 Qurb al-Isnad p. 161

Bizanti said: I asked Imam al-Ridha (peace be upon him) about a man who has a wife. Can he enter into a temporary marriage with her sister? He said: No.

4 عن أَبِي جَعْفَرقَالَ: قَضَى أَمِيرُ الْمُؤْمِنِينَ فِي أُخْتَيْنِ نَكَحَ إِحْدَاهُمَا رَجُلٌ ثُمَّ طَلَّقَهَا و هِيَ حُبْلَى ثُمَّ خَطَبَ أُخْتَهَا فَجَمَعَهُمَا قَبْلَ أَنْ تَضَعَ أُخْتُهَا الْمُطَلَّقَةُ وَلَدَهَا فَأَمَرَهُ أَنْ يُفَارِقَ الْأَخِيرَةَ حَتَّى تَضَعَ أُخْتُهَا الْمُطَلَّقَةُ وَلَدَهَا ثُمَّ يَخْطُبَهَا و يُصْدِقَهَا صَدَاقاً مَرَّتَيْن

📚 Al-Kafi Vol. 5, p. 430

Imam al-Baqir (peace be upon him) said: The Commander of the Faithful (peace be upon him) ruled regarding two sisters, one of whom a man had married and then divorced while she was pregnant, and then before her divorced sister gave birth to her child, he proposed to the second sister and combined the two. He commanded that he should separate from the second sister until the first divorced sister gives birth to her child, and then (from the beginning) he can propose to the second one and give her a new dowry.

✅The 'iddah' (waiting period) of a pregnant woman is until she gives birth to her child.

5 عن يونس قَال: قَرَأْتُ كِتَاب رَجُلٍ إِلَى أَبِي الْحسن الرَّجُلُ يَتَزَوَّجُ الْمَرْأَةَ مُتْعَةً إِلَى أَجَلٍ مُسَمًّى فَيَنْقَضِي الْأَجَلُ بَيْنَهُمَا هَلْ يَحِلُّ لَهُ أَنْ يَنْكِحَ أُخْتَهَا مِنْ قَبْلِ أَنْ تَنْقَضِيَ عِدَّتُهَا فَكَتَبَ لَا يَحِلُّ لَهُ أَنْ يَتَزَوَّجَهَا حَتَّى تَنْقَضِيَ عِدَّتُهَا

📚 Al-Kafi Vol. 5, p. 431

Yunus said: In a letter from a man to Abu al-Hasan (peace be upon him), I read: A man enters into a temporary marriage with a woman for a specific period, and their time expires. Is it permissible for him to marry her sister before her sister's 'iddah' is over? The Imam wrote: It is not permissible for him to marry her until her sister's 'iddah' is over.

✅In many other narrations as well, regarding combining two sisters in marriage, there is no difference between permanent and temporary marriage, and even marriage with a slave girl, and all of them are considered 'combining' and the rulings of combining apply to them.

✅Regarding our question as well, combining two 'sayyidah' women in marriage is absolute and includes both a permanent wife and a temporary wife. Acting upon this narration requires that if a person has a 'sayyidah' wife in his marriage, he should avoid a permanent or temporary marriage with a second 'sayyidah' woman and if he wishes to revive the 'sunnah' of temporary marriage, he should use non-'sayyidah' women, Insha'Allah.