I have had sex with two women, taking turns while one watched. Is this permissible?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is not permissible to have sex with one of your wives in front of the other, since such a situation must be concealed even from a child. A true believer should avoid such an action. The Prophet (Sallallahu Alaihi wa Sallam) was used to washing his private parts and making Wudu before he entered the room of each one of his wives.
The Prophet (Sallallahu Alaihi wa Sallam) said: "Iman (faith) consists of more than seventy branches and al-Hayaa (modesty, bashfulness -) is also a branch of Iman" . [Reported by Imam Bukhari ]
So, if your question means that you have sex with them one after the other, there is no problem provided that no one sees the other in that situation.
Allah knows best.