The objective of this game is exploration and collecting effects in order to achieve an ending. Enter Katsuki or Shun's dreams by sleeping in their bed, save your game by interacting with their desks. To switch between the protagonists, interact with the telephone in their rooms when they are awake. Press 5 to dispose an effect at the nexus like in Yume Nikki.
It is unclear what is the relation between Katsuki and Shun. Judging by the title, "Again, sometime", it could be that they were separated once, and they wish to meet again sometime. Both Katsuki and Shun refuse to leave their room/apartment. Both of them explore their dreams, are male, have short black hair, and are sentimental.
They could be brothers, judging by their appearance. They could be (childhood) friends who were set apart by life circumstances. That could be implied by the fact that you switch between them using the similar telephones in each of their rooms, also hinting us that they are now connected by phone calls.
Judging by their similar appearance and that they are sharing the same mood status - sentimental, they even could be the same person. He can either be one who decided to change his name when he grew up, or one that has split personalities named Katsuki and Shun.