Fan Casting Twin Threesome Fantasy in Sort by Tropes

Bob is attracted to Alice. By some process similar to Oven Logic, he imagines that two Alices must be twice as sexy as one. Sometimes Bob is just fantasizing, but often there's an actual second "Alice" floating around. This may be a genetic twin, a product of Applied Phlebotinum, or more rarely a sibling, parent, cousin, or even just someone unrelated who dresses and acts like Alice. For whatever reason, a fetish for twins is regarded as G-rated, so characters — especially male ones interested in female twins—can admit to it and not be teased, except for being a general horndog. A woman fantasizing about two attractive male twins however...well, you know.