Shimaidon or Shimai Donburi is a Japanese term describing a situation in which someone manages to have his/her way with two sisters at once (most often, in a sexual way). In this case it applies if when the game ends, the protagonist is in a relationship with two sisters at the same time (with both of them aware of it).

It is better (but not required) to have h-scene(s) including both the two heroines and the protagonist at once.