Unfortunately, the only things I managed to do this for were two Jessica facts from book 2. They are below. (The other things I flagged were songs to look up, outfits I wanted to see what they looked like IRL, etc.)
Jessica tried to be friends with Enid first! Part of the reason she was annoyed with her was that she was friends with Elizabeth instead. (Source: Chapter 3 of Secrets.)
Jessica is allergic to gardenias. (Source: Chapter 6 of Secrets.) I wonder if Liz has this same allergy?
Anyone got any others? For anybody in the series, not only the twins? I swear, I'm 30, been a fan off and on since I was 8, and was still very surprised by the Jess and Enid thing. Somehow I never remembered it!