Two partners are not under an obligation
to pay individually until each one of them possesses the nisab
Imam Shafi
Combined wealth has the hukm of a single individual
Similar to the mixing of the two capitals
Tradition of Abu Said can be understood to mean that the hukum applies to it when it is owned by one or more.
Abu Sa'id al-Khudri reported Allah's Messenger (may peace be upon him) as saying: No Sadaqa is payable on the grains and dates till it (comes to the Weight) of five wasqs, or less than five heads of camels, or less than five uqiyas (of silver).