How to start fair treatment between two wives

Ibn Qudaamah said: If he marries a virgin, he should
stay with her for seven nights, then start alternating (between his
wives). If he marries a previously-married woman, then he should stay
with her for three nights. That is because Abu Qulaabah narrated that
Anas (may Allaah be pleased with him) said: “It is part of the Sunnah,
if a man marries a virgin after marrying a previously-married woman,
that he should stay with her for seven days, then start sharing his
time, and if he marries a previously-married woman, he should stay with
her for three days, then start sharing his time.” Abu Qulaabah said:
If you wish, you may say that Anas attributed this to the Prophet

(peace and blessings of Allaah be upon him) and it is agreed upon. 
If the previously-married woman wants him to stay with her for seven
days, then he should do that, and make it up to the other wives later
on , because Umm Salamah narrated that the Messenger of Allaah

(peace and blessings of Allaah be upon him) stayed with her for three
days, and he said, “You are not unimportant; if you want I can stay
seven nights with you, but if I do that, I have to do likewise with
the rest of my wives.” According to another version, “If you wish, I
will stay with you for three nights, then I will alternate among you.”
According to another report: “If you wish, I will stay with you for
three nights exclusively for you.”