From MangaHelpers:

The Nahara family and the Aota family children have been friends since they were little, and now they share a secret. They appear to be next door neighbors, but they actually live in the same house! Their architect parents left them in a house that looks like two houses on the outside, but which is actually joined inside. Now Suzuna Nahara and her little brother Fuki, and Touka Aota and her little sister Tamaki share a lively daily life that they have been instructed to keep a secret from all outsiders!