I'm sure Yoshi wouldn't mind carrying two. Just this once.
This is what happens when Yoshi gets the powerup.
Yoshi had to be in therapy for months after they rode him.
He accidentally jumped off a cliff later that night.
Great, now we can't go to the party at the Haunted House.
At first it may not seem like it makes sense, but they both went to college in Yoshi Island.