These all happen in the Saturday Supercade TV show. To read about other interactions in that show see Saturday Supercade links.
September 17, 1983
Two animated shows based on the Donkey Kong series, called Donkey Kong and Donkey Kong Junior, and one based on the Pitfall series, called Pitfall! were produced and aired together in the Saturday Supercade. Additionally, since Pitfall! aired right between Donkey Kong and Donkey Kong Junior, the show also featured transition bumpers between them, showing characters from both series meeting each other. These are specifically:
Donkey Kong to Pitfall! transition
This clip shows Donkey Kong swinging on vines in the jungle, then Pitfall Harry's pet mountain lion Quickclaw is seen resting on a branch. Donkey Kong accidentally grabs his tail, making him scream, then smashes himself against Pitfall Harry, that was swinging in the opposite direction, and all three then fall to the ground.
Pitfall! to Donkey Kong Junior transition
This clip shows Pitfall Harry swinging on vines, followed by his niece Rhonda and then his pet mountain lion Quickclaw, that smashes himself against a tree and then falls down right into the sidecar of Bones, where Donkey Kong Jr. was sitting.
Ruby-Spears was licensed by the games' respective companies (Nintendo and Activision) 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.