What is the ruling on wealth that is partially mixed and partially held separately?

Primary text

If a person's wealth is partially mixed with another's and partially held separately, the entire wealth is treated as mixed, provided the portion that is mixed reaches the Nisab (threshold for Zakat). If the mixed portion is less than the Nisab, the ruling of mixture does not apply. When mixing occurs such that the total combined wealth reaches the Nisab, Zakat is due based on the proportion of ownership. For instance, if one man owns sixty sheep, twenty of which are mixed with another man's twenty, one sheep is due from both, distributed according to their shares of the total eighty sheep.