
So when you scan the yarn Yoshi while playing the game, another Yoshi will join you, so it's like playing the two player mode with one player.
On the map screen you may have noticed an amiibo hut. If you scan the yarn Yoshi in there, you'll be able to play as that color in the main game if you pick it.
Do you have any other amiibo figures? If so, and if they were released before or around the same time as the Yoshi's Woolly World game, you'll be able to get playable yarn Yoshi's who have the same pattern as that amiibo you scanned. There's nothing like playing the game as a Mega Man Yoshi!