When you can't decide between two things, so you and another person (usually a friend or acquaintance) each purchase one of the desired items and you both exchange half of each item with one another.
Note: Most often done when ordering food. This way you can eat both items you want instead of ordering two meals and looking like a fat ass.
Person 1: "Man, I can't decide between the teriyaki chicken or the club."
Person 2: "Yeah, me neither."
Person 1: "Double halfsies?"
Person 2: "You read my mind."