وَفِي سَبْعِينَ: مُسِنَّةٌ وَتَبِيعٌ،
Translation: At seventy cattle, one musinnah and one tabi‘ are due.
70 = 40 + 30, so one musinnah and one tabi‘ are due. These belong to different age classes: two completed years and one completed year, respectively.
The scheduled animals define the due categories; they are not an unrestricted allowance to provide any two cattle of any age.