Dr. Monzer Kahf, Islamic Economist & Financial Expert, he said,

For the qurbani, it is one sacrifice for the household. This sacrifice can be any number of sheep, goats, cows or camels; it does not have to be one.

For zakah, if the properties are all mixed, they just calculate Zakah on these properties including bank accounts. Joint account means ownership on the basis of half/half. And zakah is due on the total amount in the account once it (not the share of each) reaches nisab.