Futakoi:

It is explained during the story that these twin girls lived a luxury life together in their mansion. Their father travels often and employs their butler to take care of them. They undergo many tests written out by their father so they are able to fit into society and live as normal high school girls. In the first episode, they are fifteen years old, but have never been in a convenience store. Unlike Sumireko and Kaoruko, Kira and Yura work together to win Nozomu's favour. [Wikipedia]

Futakoi Alternative:

Kira Sakurazuki is one of the twin granddaughters of the local yakuza boss. Occasionally the twins dress up as masked heroines battling against the yakuza's various underground activities. [Wikipedia]