Once you're done with your second playthrough to mop up your remaining achievements, simply start up a new game. Then walk up to the TV in Mickey's room and press
, collect the hat on the table and follow the brooms up. Now, collect the brush and go through Yen Sid's Lab. The next level is Dark Beauty Castle. Play through this normally until the end. If you're alone, simply go solo. Oswald will follow you naturally. For this achievement, you simply have to finish the level with two players. As long as Oswald is controlled by a second controller when the fireworks are lit, you'll unlock the achievement.