All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no doubt that it is an obligation on the husband to be just with his wives. Being just is the condition by which it is permissible for a man to take more than one wife. Allaah The Exalted says (what means): {…then marry those that please you of women, two or three or four. But if you fear that you will not be just, then [marry only] one.} [Quran 4:3]
Being just with wives in terms of financial maintenance means that he must give each enough to cover her basic needs. Beyond those basic needs, he has the right to give whatever gifts he wishes to any of his wives and he is not obliged to give a similar gift to each wife. Please refer to Fataawa 91417 and 174179.
However, the husband is advised to be thoughtful; whenever he gives any of them a gift, he should be considerate of the other wife's feelings and avoid making her sad. This is a part of being kind to the wife, as is legislated. Moreover, it was the practice of the Prophet, sallallaahu ‘alayhi wa sallam, to have consideration for the feelings of others. ‘Abdullaah Ibn ‘Abbaas narrated that As-Sa‘b ibn Jaththaamah Al-Laythi gave some zebra meat to the Prophet, sallallaahu ‘alayhi wa sallam, while he, sallallaahu ‘alayhi wa sallam, was in either Al-Abwa’ or Waddaan. The Prophet, sallallaahu ‘alayhi wa sallam, declined to accept it. When he saw the sadness on Al-Laythi's face, he said to him: "We declined to accept your gift only because we are in the state of Ihraam (ritual state of consecration)." [Al-Bukhari and Muslim] If this is the Islamic attitude towards people in general, then how about one's wife and life partner to whom he is bonded by a solemn covenant! Allaah The Exalted says (what means): {And how could you take it while you have gone in unto each other and they have taken from you a solemn covenant?} [Quran 4:21]
In any case, the husband should try his best to treat his wives fairly in everything. Marriage should be governed by mutual respect and understanding. Also, co-wives are advised to strive to maintain good relations as this helps both of them gain the husband's affection. They should also try to strive towards the family's stability and a good upbringing of their children. For more benefit, please refer to Fatwa 91150.
Allaah Knows best.