also: ふたりづつ
adverb, noun which may take the genitive case particle 'no'
two by two; in pairs; as a couple
Describes doing something in groups of two, or distributing items two per person/unit.
生徒たちは二人ずつ並んで歩いた。
The students walked in pairs.
このプリントを二人ずつに配ってください。
Please hand out two copies of this handout to each pair.
Means 'one by one' or 'one each', whereas 二人ずつ is specifically for groups of two.
The distributive suffix; 二人ずつ is a specific combination meaning 'two each' or 'in twos'.
Compound of 二人 (ふたり, 'two people') and the distributive suffix ずつ ('each', 'at a time').