If a person owns gold and silver, but neither of them on its own constitutes a nisab, he should not combine the two in order to obtain a nisab. This is because they are not of the same kind. The basic rule is that no category can be combined with another. It is the same for cows and sheep. For example, if someone has 199 dirhams and nineteen dinars, he is not supposed to pay zakah on them.
Section : Combining Gold and Silver.