Futakoi:

Twin girls who are Nozomu's childhood friends. They also attend the same school as Nozomu throughout the anime. They both harbor secret feelings for Nozomu. In kindergarten, they said they would both marry him. Although they are close, they keep their feelings secret from each other and everyone else. [Wikipedia]

Futakoi Alternative:

Kaoruko Ichijō is a shrine maiden along with her twin sister Sumireko. [Wikipedia]