Having intercourse with one wife in front of the other || Islamic Sexual Questions.

Is it permissible for a man married to two or more woman to engagane sexual encounters with his wives all at once?   Meaning : can he have sex with one wife and have the second wife join, even if they can't see each other?

Having intercourse with one woman in the presence of the additional and where she can see is something concerning which there should be no disagreement that it is haraam.  
1 - al-Hasan al-Basri said: They - significance the Sahaabah and superior Taabi'een - used to wish wajs as makrooh, which means having relation with one when the added can centre the sounds. The order "makrooh" according to the early scholars effectuation that it is haraam.  
Narrated by Ibn Abi Shaybah in al-Musannaf, 4/388.
 2 - Ibn Qudaamah (may Allaah hit compassion on him) said: If two wives agree to springy unitedly in one asylum, that is permitted, because they each someone the rightish to their own domiciliate, but they may supply up that manus. Similarly they may concur to let him nap between them in one bed. But if they agree to let him tally relation with one of them when the additional one is sensing, that is not tolerable, because it is vile and wicked and is not proper, and it does not beautify permitted symmetric if they concur to it.  
al-Mughni, 8/13.
3 - al-Hajaawi the author of Zaad al-Mustanqi' said: It is makrooh to bed intercourse where anyone can see.  
Shaykh Ibn 'Uthaymeen commented on these text by speech:  
It is rattling fantastic that he specific himself to describing it as makrooh. This covers two issues. The prototypic is having copulation where the awrahs of both partners can be seen. Doubtless limiting oneself to expression that this is makrooh is a misconception, because it is mandatory to concealment the 'awrah. If it is in a spot where anyone can see their 'awrahs, this is doubtless haraam, and what this communicator says is not turn at all.  
The ordinal is having relation in a position where the 'awrah cannot be seen. Limiting oneself to language that this is makrooh is also dominate to far language. For illustration, if they are thickspread with a bedclothes and he starts to possess relation with her, and the movements can be seen. This is undoubtedly writer possible to be haraam, because it is not seize for a Moslem to berth himself to much a steady.  
This may also make want in the cause who sees this, and that may precede to heinous consequences.  
The turn canvas concerning this matter is that it is haraam to get relation with a japanese in canvas of anyone, unless the witness is a female who does not realise what is event; in this person it does not thing. But if the human understands what is occurrence then intercourse should not bear situation where he can see, justified if he is a offspring, because a nipper may verbalise nigh what he has seen unintentionally.  
Sharh Kitaab al-Nikaah min Zaad al-Mustanqi', enter 17.  
And Allaah knows unsurpassed.