Is it permissible to engage in sexual intercourse with one's wives in the same room, where one sees the other, with their consent? Is there any explicit text in the Qur'an or Sunnah that prohibits this, or does this prohibition fall under the category of manners and ethics?

It is forbidden for a man to have sexual intercourse with his wife in the presence of another [woman] if it leads to her seeing their 'awrah (private parts). It is discouraged from the perspective of good manners even if it does not lead to that. Observing good manners and morals is important, for the Prophet, peace and blessings be upon him, said: "I have only been sent to perfect good character," and "If you have no shame, then do as you please."