Wow, that is him, isn't it? I never once thought "Oh that's the guy who did Johnny Bravo" while watching Flapjack, but now that you mention it, I can totally hear him in there. He's Peppermint Larry (amongst others probably), right?
Also, I thought of one. I would guess they've probably worked together on more series than the two I'm thinking of, but: Maurice LaMarche and Frank Welker as Egon and Ray on The Real Ghostbusters, then as various characters (such as Morbo and Nibbler) on Futurama. Though, I'm not sure that any of their Futurama characters ever spoke directly to each other.