What is the ruling on one of the two wives sending text messages to her husband outside of her designated time, for non-essential matters, while she already knows his whereabouts?

A woman should not do that on her co-wife's night unless there is a necessity or need. The wife whose turn it is has the right for her husband to spend the night and day with her. The jurists have prohibited the husband from going to the other wife on her co-wife's night without necessity, or during the day without need. Praise Allah for a righteous husband, and beware of what sows hatred between you.