Complex sentence: Hanna and Barbera, contemplating their future while completing the final "Tom and Jerry" cartoons, began producing animated TV commercials and during their last year at MGM, they had developed a concept for an animated TV program about a dog and cat in various misadventures.
Split into simpler sentences: Hanna and Barbera, contemplating their future while completing the final "Tom and Jerry" cartoons, began producing animated TV commercials. During their last year at MGM, they developed a concept for an animated TV program about a dog and cat in various misadventures.