OP here--thank you for all the replies. These are our first kids applying to college so it is a bit daunting. I'm not the most organized person and of course teens are, well, teens. I think the most important thing is for us to try to minimize feelings of unhappiness or self-doubt, especially with my twin who is weaker academically and tends to give up when they think things aren't going their way. So some of the advice here about making sure to aim for different schools and other posts I have seen about rolling admissions to build confidence are helpful thank you!

I think part of what's tough about assessing the twin factor for highly rejective schools is the same as any other pairing of qualified applicants: you don't know the school's institutional priorities. In our case, our daughter had better grades (though both kids were strong, and both with highest rigor at their school) and had a better SAT (again, scores were close), … but was a girl; our son had slightly better ECs, probably slightly better LORs, and is a boy. So … different schools could have seen either one as "stronger" depending on their institutional priorities.

In our case, the admission results (for both) perfectly mapped to overall school rejectivity — any schools under a 5% admit rate = rejection, any schools between 5 and 15% = waitlist, any schools over 15% = admit, and any schools over 30% = admit with merit money. That was true regardless of whether they both applied to the school or only one did.

We were honestly grateful that both had the same acceptances. We would rather that both twins go to an amazing-if-not-tippy-top school than for one to have ended up where they did and for one to have ended up at a HYPMS.

I am the 15:58 poster and should clarify - both twins were strong in general, but one was stronger for their preferred major/program than the other. So for example one twin might have been in the 75th percentile, the other the 50th percentile. In most cases that would be fine, but at some point it could knock one out.

I also know a few admins at campuses and they have told me their preference is not to split. It happens, but it is not as common as you might think. The stronger one would have to be very strong to pull in another.