What is the religious ruling on someone who had intercourse with his two wives simultaneously, being ignorant of the act's prohibition, and one of the wives disclosed the matter to her family, leading to family problems and their demand for divorce? What should he do?

It is forbidden for a man to have intercourse with his wife in a manner where another woman can see them both undressed, due to the exposure of the severe 'awrah (private parts). If their 'awrah is covered, scholars have differed on the ruling, between prohibition and dislike. The preponderant view is absolute prohibition, as it involves debasement, lack of chivalry, and incitement of hatred between co-wives. Therefore, having intercourse with both wives simultaneously is forbidden, and one must repent to Allah. As for the wife informing her family of the details of what occurred, it is forbidden, unless she was compelled and could not deter her husband except by informing them in a general way. You must apologize to your in-laws for your ignorance of the Sharia ruling and promise them that it will not be repeated.