ずつ and ごとに: shares and recurring intervals
Distinguish each person's share, gradual progress, and repetition at regular intervals.
ずつ gives the amount for each person or step
After a quantity, ずつ indicates equal distribution or handling a certain amount at a time. 一枚ずつ配る means to hand out one sheet each or at a time; it does not mean that there is only one sheet in total.
参加者に資料を一枚ずつ配りました。 means that one sheet of material was handed to each participant. 参加者に identifies the recipients, while 一枚ずつ specifies each person's share.
ごとに sets the unit of repetition
バスは十分ごとに来ます。 means that the bus comes every ten minutes. 十分ごとに sets the interval between arrivals, not a ten-minute stay each time a bus arrives.
毎日十分ずつ練習します。 means to practice for ten minutes each day. 十分ずつ gives the amount of practice allocated to each day; 十分ごとに練習します would describe a different schedule, with practice recurring at ten-minute intervals.
Recognize gradual progress too
毎日少しずつ庭の草を抜いています。 means to pull weeds from the garden a little at a time every day. 少しずつ presents small amounts accumulating rather than doing everything at once, but it does not specify exactly the same number of weeds each time.
First ask whether a quantity is a share or a recurring interval. 二個ずつ means distributing or handling two at a time, while 二日ごとに means repeating something every two days. Treating both as a single general marker of distribution can hide the timing difference.
In 一人に二枚ずつ配ります and 二日ごとに配ります, what do 二枚 and 二日 specify?
Show the answer and why
二枚ずつ specifies a share of two sheets for each person. 二日ごとに specifies that distribution takes place every two days; it does not say how many sheets are handed out each time.