It was Starsight (Skyward series, book 2). The diones are apparently genderless or have unknown genders, and reproduce by merging into a single being for a while so they can "try out" the adult version of the kid they're going to have. If they don't like the kid they can reject that version and try again (eugenics), otherwise the actual kid is born from an egg or something.
The whole genderless thing was completely irrelevant to the plot—the diones could have been male or female and little else would have changed. It's not that it made me uncomfortable, but it was an unnecessary distraction.
Anyway, the book was a 2/5 because it threw almost everything from book 1 away, had way too many characters and places, ended on a serious cliffhanger, and the fictional science was extremely far-fetched. Meanwhile, book 1 was fantastic (a full 1/1 in my opinion) was focused on a small cast of characters on a small set, had great character development, and resolved nicely.
