also: ふたつづつ
adverb, noun which may take the genitive case particle 'no', noun
two at a time; two by two; in twos
Indicates that something is done or distributed in groups of two. Can be used adverbially or with の to modify a noun.
子どもたちは二つずつお菓子をもらった。
The children received two sweets each.
二つずつのグループに分かれてください。
Please split into groups of two.
The suffix ずつ attaches to counters and numbers to mean 'each' or 'at a time'. 二つずつ is a specific instance with the counter 二つ.
Compound of 二つ (futatsu, 'two') + ずつ (zutsu, 'each' / 'at a time'). The suffix ずつ is of uncertain origin but is used to indicate equal distribution or repetition.