After working together on Scooby-Doo, Smurfs, and other projects for 15 years, Don Messick & Frank Welker shared the screen together in an episode of Don's NBC primetime series, The Duck Factory.
In "The Duck Stops Here", Wally Wooster (Messick) is afraid he's lost the voice of Dippy Duck. Worse, the studio brings in another actor (Welker) as a potential successor. Ironically, 18 years later, Welker was finally given the green light to be the voice of Scooby, 5 years after Messick had passed away.
Frank arrives about halfway through.
Keep an ear for Don's dramatic turn as he recites some Shakespeare, to the surprise of his co-workers, Skip & Brooks (Jim Carrey & Jack Gilford).
To be perfectly honest, I think Don wanted Frank to take over as Scooby all along if anything happened to him, but WB didn't get the message, which is why it took 5 years before the torch was passed.