Violette and Hortensia are Hiver Laurent’s twin dolls who represent night and day as well as death and life respectively. In order to find the meaning of life, Hiver searches for stories that involve life and death.
Sound Horizon’s most popular album, "Roman" is a collection of such stories where each song represents one romantic story.
Aren’t they beautiful? They really look like dolls, though maybe not twins. Great cosplay!
EDIT: Thank you to Koneko for letting me know who the cosplayers are! Violette is cosplayed by Saya and Hortensia is cosplayed by Tsugihagi Mishin from Japan!