Dr. Monzer Kahf, a prominent Muslim economist and counsellor, states that: “For example, if a husband and wife mixed their account and made it joint, I tend to apply the rule of mixed property on them. Zakah is due on the total money the moment it reaches nisab and a lunar year has passed.
They can divide the liability between themselves in proportion to their ownership. This applies to all such mixing of properties, not only on husband-and-wife bank accounts. The International Shari`ah Board of Zakah has adopted such a view with respect to Zakah on companies. The source of this ruling is the Hadith of the Prophet (peace and blessings be upon him) with regard to mixed livestock, which provides the same rule.”