Rikkun, Game Umai ne. Kakkoii ne.

A boy who's good at games gets two busty older girls as his disciples! But when he tries to teach them his special techniques, he cannot concentrate because of their sexy bods. They notice that's he's been sneaking peeks at their titties, and start to tease him about it. And then..! (Source: DLsite, official) Note: Released at Comiket 96.