Yep, both characters, Kaoru Hitachiin and Shion Sonozaki have identical twins.
Perhaps to avoid adding unnecessary difficulty, we could name the anime the characters come from?
But how general can it end up. I mean those last two also both have computer aided artwork for the environments.
Both also aired in 2006 and feature VA Yukari Tamura.
I wouldn't consider something as commonplace as computer generated artwork or a series having had the some of same staff working on it as as true connection though. I guess what I really should have titled this is "Guess the anime
character connection".
Obvious common traits like:
#Both characters practice kendo ~ Tenshi, Kuno
#They're both gender-benders ~ Ranma, Maze
#Both characters share the same actor ~ Akane, Kikyo
#Blond anti-heroines who die in the first episodes of their own series ~ Excel, Seras Victoria
#They love curry ~ Ciel, Kaolla Su
#They're both old massively perverted martial arts masters ~ Happosai, Muten Roshi