A term common in Chicago for a multi-story house that's been remodeled to accommodate an apartment on each floor. Sometimes also referred to as a "polish flat," likely referring to Chicago's large Polish population.
Dude 1: Hey, should I get an apartment in a high rise or a two flat?
Dude 2: Just get an apartment in a two flat. It's less expensive and you won't seem like a yuppie douchebag.