I remember a lot of two sided DVDs with early gen players. I'm not sure if it was before double layer or just worked on more players then going double layer, either way, it's nothing new.
Once Upon A Time in America, a film renowned for its sophisticated editing and treatment of intricate timelines, is split across two DVDs requiring you to change discs in the middle of the factory action sequence.
Well since no-one's mentioned it - with the controversy on the new, high density formats for the DVDs (Blueray and HDTV?), some distributors are releasing disks with one format on one side and the other on the reverse.