这个回答比较靠谱搜到两个佐证，MOE是说treated as one application，Strait times则明确说allotted one number。
MOE:
Question:
Will my children who are registering at the same school all be admitted if one of them is successful in a ballot?
Answer:
Siblings, twins or triplets of the same citizenship will be treated as one application for the purpose of balloting. This means that all children will be admitted to the school if one of them is successful in getting a place after the computerised balloting.
Strait times:
Q: I am registering my twins for Primary 1 this year. If there is balloting, what happens if one gets in and the other does not? Or will they be balloted together?
A: Yes, if there is balloting, your twins will be allotted one number. If it is picked, both your children will be admitted to the school.
Kiasu上的一个帖子
2011年的，不知道准确性如何。
Yes for twins/triplets, they can choose to either ballot individually (means each child gets 2 slips) or collectively (means 2 slips for each set of twins/triplets).
For the former, only the child that gets picked will get a place and the twin that doesn't get picked will have to go to another school. For the latter, the whole set of twins/triplets will go through. So you see, it is a calculated risk on the part of the parents, they get fewer slips but if they win, they really win big!
If these twins/triplets were to be picked last then the school will have to create more places for them but since they weren't, they took up 7 out of the seats alloted for P2C. Like in St Nick's Girls at P2B, the twins were picked last and the school had to take an extra seat from P2C to give it to them at P2B.
Yes for twins/triplets, they can choose to either ballot individually (means each child gets 2 slips) or collectively (means 2 slips for each set of twins/triplets).
For the former, only the child that gets picked will get a place and the twin that doesn't get picked will have to go to another school. For the latter, the whole set of twins/triplets will go through. So you see, it is a calculated risk on the part of the parents, they get fewer slips but if they win, they really win big!
If these twins/triplets were to be picked last then the school will have to create more places for them but since they weren't, they took up 7 out of the seats alloted for P2C. Like in St Nick's Girls at P2B, the twins were picked last and the school had to take an extra seat from P2C to give it to them at P2B.
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