Commentary for Kiddushin 22:2

The reason why the claim must be for two pieces of silver and not just one is a midrash comparing money and utensils, which appears in the plural. Just as the claim must be for “utensils,” i.e. a minimum of two, so too must the claim of money be for at least two coins. These coins must have value, like utensils. So it cannot be for two perutot, which have minimal value.
Thus this source does not disprove the rule that any money mentioned in the Torah must be Tyrian coins.

Second tithe is produce which is then redeemed for money and the money is brought to Jerusalem where it is used to buy food. The fact that second tithe is mentioned in the Torah implies that the minimum amount should be a dinar. And nevertheless, there is a mishnah that refers to a sela of copper coins, which is a lesser amount.
The Talmud resolves that this is a special case—the Torah states “the money” and not just “money” to teach that even a lesser amount would be valid as second tithe.