All praise be to Allah, peace and blessings be upon His Messenger: Our master, Prophet Mohammad.
There is no doubt that in principle a joint bank account should be divided according to the manner agreed upon by the couple. Therefore, if one of them passes away, the other takes his/her share accordingly, and the remainder is divided among the heirs in line with the Prophet`s Hadith: “ A true Muslim is bound by the terms that he approved of. “{Bukhari}.
In case of the husband`s passing away, the wife takes her share from the money in the aforementioned account, and the remainder is divided in accordance with the rules of Sharia (Islamic law) where she gets a fourth if the husband didn`t have a son, and an eighth if he did since Allah, The Almighty, says in this regard: “ye get a fourth; after payment of legacies and debts. In what ye leave, their share is a fourth, if ye leave no child; but if ye leave a child, they get an eighth; after payment of legacies and debts. If the man or woman whose inheritance is in question, has left neither ascendants nor descendants, but has left a brother or a sister, each one of the two gets a sixth; but if more than two, they share in a third; after payment of legacies and debts; so that no loss is caused (to any one). Thus is it ordained by God; and God is All-knowing, Most Forbearing.” {An-Nisa/12}.
What remains is divided on the heirs, each according to his/her share. However, if the couple had no prior agreement, then the regulations of the bank itself regarding such accounts, and which were signed by the couple, are to be put into effect.
Hasn`t there been any bank regulations for such accounts, then the money is equally divided between the couple. And Allah knows best.