It was typically day schools. My assumption is that in a BS situation is that situation is not that impactful given they are residing at the school.
This just had me think of another situation. Has anyone know of a case where one twin is accepted and not the other one. Now that is a corner case if I ever heard of one. 
@twinsmama had one twin accepted. The other twin reapplied the next year and was accepted. Our tour guide at one of the schools was a twin who also had to reapply and was accepted the year after her twin was accepted. It happens.
DS had a freshman roommate who was a triplet. Sister went to a different school, and the brother came to DS school in 10th grade. I don’t know who applied where for 9th grade. I think maybe the one who came in 10th wasn’t ready to go away in 9th.
I know a family whose twins ended up at 2 different schools, but I think it was deliberate based on fit. One went to a school where an older sibling had gone. Actually, DS had quite a few classmates with siblings at other schools, often at the same time. While vacation schedules may not always mesh perfectly, BS really offers a great way to get the right fit for each child. I know 2 kids who wanted to avoid a school where they might be so and so’s sib.
At one point, there were triplets at our school, so it really depends on the students, the family, and the school.
I also have twin nieces who chose to apply to some different schools and ultimately attend different BSs as well as now different colleges.