Not quite a double date, but not exactly a date either. It's when you invite someone out on a date, and they pull a defensive maneuver and bring a friend along, as a date shield.
Dude, I asked out that hot chick Sally on a date Saturday to cool party. She said yes, because she knew it was a cool scene, but she turned it into a one-and-a-half date, bringing along her BFF Tina to cock block.
A twosome would involve two normal-sized people, so therefore a one-and-a-halfsome would involve a normal-sized person and a person half the size of a normal person, or a midget.