These all happen in the Saturday Supercade TV show. To read about other interactions in that show see Saturday Supercade links.
September 08, 1984
TV shows based on both series, called respectively Donkey Kong and Kangaroo, were produced and aired together in the second season of Saturday Supercade. Additionally, since Donkey Kong aired right after Kangaroo, the show also featured transition bumpers between the two, showing characters from both series meeting each other.
Ruby-Spears was licensed by the games' respective companies (Nintendo and Atari) to produce their cartoons. Once they got the license for both series there was nothing preventing them to make characters from both series appear together.