Is it permissible for a second wife to give up some of her nights to the first?

This involves an agreement between a man and his [second] wife that she releases him from a share of what is due to her. Thus, if he is already married, his second wife could willingly forego her right to spend the same number of nights with him as her co-wife. There is nothing wrong with forgoing one’s rights.
One of the Prophet’s wives, Sawdah, gave her turn to Aisha and the Prophet (peace and blessings be upon him) accepted this arrangement. This is entirely up to the parties concerned. The only thing is that there should be no pressure on a wife to forgo what is due to her by right.