Innie & Outie is a short film that is listed on the Nickeldeon Creative Lab Comp Reel. It may have appeared as one of the Nickelodeon's Short Films by Short People series; a program that showcased short films made by kids. It was created and aired in 1996, and may have been shown for a few years after that date, often during the space between Nick Jr. and the normal Nickelodeon programming block.
Its description is listed on the tape as a story "about two friends whose faces form a belly button, as they go to a movie for the afternoon." The short featured human actors with faces drawn on their chests, likely in sharpie, with the human actors moving their stomachs to simulate speech and other actions, such as drinking soda from a straw. The main characters were archetypal 1990s teenagers and the film was set in a city.
The Paley Center for Media  in California were confirmed as having a copy of Nick's Creative Lab Comp Reel (containing Innie & Outie), and a screening was organised to take place on February 17, but like that day's screening of the First Ever Short Films By Short People Festival, it went rather unsuccessful. Its listing on their website is one of the only sources proving its existence, outside of a resumé for Howard Hoffman  and a plethora of Yahoo Answers threads inquiring about the video.
In November 2016 a youtuber under the name of 'misslizzie2007' uploaded a commercial break which had Innie & Outie in it, which meant the short had been found.