Is it permissible for a man to have sexual intercourse with his two wives at the same time, without either of them seeing the other?

It is not permissible to have intercourse with two wives at the same time in a way that each sees the other, due to the prohibition of each seeing the other's nakedness (awrah). It is also discouraged even if there is concealment and no risk of seeing the nakedness, as a matter of good manners, and because it incites jealousy and disagreement between them.