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.
It is forbidden for a woman to ask her husband to divorce her co-wife. It is narrated on the authority of Abu Hurayrah that the Prophet, sallallaahu ‘alayhi wa sallam, forbade that a man should propose to a woman who is engaged to his brother, or override his transaction, and that no woman should ask for the divorce of her sister (i.e. co-wife) in order to take her place, for her sustenance is incumbent only on Allaah The Almighty. [Al-Bukhari and Muslim]
This Hadeeth (narration) restricts the general significance of the statement of the Messenger of Allaah, sallallaahu ‘alayhi wa sallam: "The conditions which are most entitled to be fulfilled are those therewith private parts (i.e sexual relations) are made lawful for you (i.e. the conditions of the marriage contract)." [Al-Bukhari and Muslim]
Based on this, the condition on which you and your wife have agreed is forbidden, and forbiddance renders it invalid. Henceforth, it is not due, if not impermissible, to fulfill it. This is relevant to the ruling on the fulfillment of conditions.
Furthermore, we bring to mind that divorce is among the rights given to the husband: so, whenever he does it, it becomes effectuated, whether or not the wife accepts it.
Allaah Knows best.