What about Sal and Hyperchicken? They'd make a great duo.
They're too near the bottom rung of the latter to match up. For it to work well I think you would need at least one of them to be a major (er) character...but tbh I don't think I could stand hyperchicken or Sal for a full episode. They work well as they are now.
I am actually surprised we haven't seen some Nixon/Mom pairings. Ha, can you imagine a Morbo/Lrrr pairing...it would be 21 minutes of Maurice going at it with himself
And then denying the two sound even remotely alike