It would have to be double Zelda or double Pikachu from my experience, assuming team attack is off. They can literally create a wall of attacks back to back from each other that makes approaches near impossible. In Zelda's case, double Din's Fire or Fsmash/Usmash, and in Pika's case, I'm pretty sure double Dsmash is possible, and double Thunder is basically a wall you're not going to cross, ever.
With Team Attack on, I wouldn't know. I would like to play those matches, but I don't get many people interested in it. :/