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.
