A Diddy/Dixie combo would have to work differently than the Ice Climbers, if only because of Dixie's hairspin.
Just look up the sales numbers for games where Diddy is the main star... DKC2, DKL2, and DKR. Those 3 games and their remakes have sold over 14.5 million copies. That's pretty similar to Metroid, bigger than StarFox, and a lot bigger than F-Zero, but people want second characters from all those franchises.
And then add the games where Diddy is a sidekick... DKC, DKL, DK64... Those were even bigger games... In total, DK games have sold around 50 million, and Diddy is in practically all of them, often playable.
With that kind of popularity, he's really hard to disclude.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.