How is Zakat calculated for two separate amounts that together reach the minimum threshold (Nisab)?

It is not permissible to deposit money in post offices or usurious banks, because the depositor there is not free from consuming usury or authorizing it. Whoever possesses money that reaches the [nisab] threshold, even if it is scattered, must pay its zakat if a year has passed over it. This is one-quarter of one-tenth (2.5%).