In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- It is to be noted that having sex with one’s wife in front of any other person, even if a second wife, is not permissible.
2- Having sex with one’s two wives together is prohibited even if they are willing to do it, as this involves allowing them to see the private parts of each other, which is not allowed in Islam. This also goes against the Islamic teachings that require that bedroom secrets should be confined to the husband and wife only. Moreover, this violates the basic norms of modesty.
Elaborating on this, Sheikh Faysal Mawlawi, the late Deputy Chairman of the European Council for Fatwa and Research, states:
It is not permissible to have sex with one’s two wives together for two reasons:
First, it is not permissible for a woman to look at the `awrah of another. The `awrah of a woman with respect to other women is what is between the navel and the knee.
The Prophet (peace and blessings be upon him) said, ‘… and a woman should not see the private parts (`awrah) of another woman…’ (Muslim and At-Tirmidhi)
Second, the Prophet (peace and blessings be upon him) prohibited the man and the woman to talk to others about details that happen in their bedrooms.
The Prophet (peace and blessings be upon him) said: ‘Do not do this. Do you know the similitude of those who do that? They are like a male and female devil who meet each other in the road and satisfy their desire with the people gazing at them.’
This indicates that with greater reason a man must not have intercourse with one of his wives in the presence of another, as the Prophet (peace and blessings be upon him) did not make exception for the second wife in this Hadith.
The Prophet (peace and blessings be upon him) was very keen not to have intercourse (with any of his wives) while anybody could see or hear them.
In his book Al-Mughni, Ibn Qudamah stated that if the two wives agreed that the husband has sex with one of them while the other can see them, this is not allowed, as this is mere degradation and absurdity that violate the rules of modesty and go against sound taste.
Allah Almighty knows best.