Is it permissible for a wife to grant her night to her co-wife? And is it obligatory to spend one night at a time, or two nights at a time?

A woman may donate her night to her co-wife, provided her husband consents. If the wife travels with her husband's permission for his need, her right to a fair share of his time is not forfeited, and he must compensate her. She may also donate her nights to someone else. However, if she travels for her own need or without his permission, she has no right to a share, nor can she make such a donation. If the wife's travel is for the husband's need, she may donate her nights to her co-wife with his permission. But if she travels for her own need, her right to a share is forfeited, and her donation is not valid.