I went to an all-girls boarding school, which had a close partnership with the all-boys school (which was co-ed in the sixth form) in the town, and there were many joint extra-curriculars outside of academics. For example, most of our school plays (but not all of them), were joint productions with the boys school.
It worked well, because we received the benefits of a single-sex education, but we regularly mixed with the boys. And once in the sixth form, you were mixing with the boys, and some new girls too (which was nice, as you got to see some new faces, after seeing the same old ones for five years)!
I don't know how the partnership works now, as the all-boys school became co-ed all the way through, the year after I left. There is nothing on either school's website to say they work together, so I wonder whether that partnership has fizzled out, now the other school is co-ed.
(I've noticed that quite a few all-boys schools have become co-ed but very few all-girls schools have).
My husband went to an all-boys day school, with a co-ed sixth form, so as we're both products of a (mainly) single-sex education it's something we always planned to do with our children. Indeed our two sons (14 and 7) were both in an all-boys school before we moved to the States. However, we're now living in the suburbs of Pittsburgh, and there are only two single-sex schools in Pittsburgh (and indeed within a 30 mile radius of where we live). These schools are Roman Catholic, and we're certainly not sending our children to those, so our sons are now attending a co-ed private school in Pittsburgh, and our oldest daughter (4) will be starting in the pre-school there in September.
Things are a bit different here though, because in the US system, you usually have to take at least three years of maths and science in high school, and if you want to get into a good college you're advised to take four years of each. In fact the school our sons are attending, you HAVE to take four years of maths and science, so our daughters (if they get places) will have to take those subjects. So there won't be any question of them feeling like they can't take those subjects, because they're seen as "male" subjects, as they'll have no choice. I kind of like that, if I'm honest.