Signboard consisting of two hinged boards that hang front and back from the shoulders of a walker and are used to display advertisements WordNet 3.0 & Open English WordNet

A sandwich board is a type of advertisement tool composed of two boards with a message or graphic on it and being either carried by a person, with one board in front and one behind in a triangle shape, hinged along the top, creating a "sandwich" effect; or set up next to a store advertising its goods. Wikipedia

A person-sized, wearable billboard or advertisement consisting of two printed boards strung together so that they hang over the shoulders, or simply set out on a street. Wiktionary