بسم الله الرحمن الرحيم
Division is of two types:
Physically is to give her a night similar to what is given to her sister wife. To each is due equity (in that regard) and injustice is forbidden. Therefore physical division is obligatory, as the Exalted says:
و لا يَجْرِمنَّكم شنآن قومٍ على ألاَّ تَعْدِلُوا اعْدلُوا هو أقرَبُ للتقوى
“…and do not allow enmity towards others cause you to be unjust. Be just! that is closer to piety…” [Al Maa’idah: 8]
Likewise the narration of Anas where he said: “From the Sunnah is that whenever a man marries a virgin (while being married; meaning he takes a second wife) he spends seven days with her, then he divides (the days between both wives). If he marries a woman who had previously been married, he spends three days with her, then divides.” Collected by Al Bukhari.
Therefore, whoever is inequitable has sinned, due to what Ahmad and the compilers of the sunan reported with an authentic chain of transmission, wherein the Prophet -sallahu alayhi wa salaam- said, “whoever has two wives and he leans towards one over the other, he will come on the Day Of Resurrection and one half of him will be leaning.” Equity in wealth is also obligatory and injustice therein is not permissible.
Translated by Najeeb Ibn Yusuf Al Anjelesi
Source: المنظار في بيان كثير من الأخطاء الشائة