Disney Infinity

This can be done with just the starter set.

First you have to have an existing save game for one of the worlds. For the example we will be using sully and the monsters university level.

If you have an existing save you can skip to the next paragraph
Place the Sully character on slot one of the base, and load up the Monsters University level. After the cutscene plays and you have control of your character, hit start and save the game. Then in the start menu click load. and load a toybox save.

From inside the toybox place another character, either Jack Sparrow or Mr. Incredible, on the second slot of the base. Then plug in another controller and hit start to join.
At this point you you should be in splitscreen mode.

Hit start on player one's controller and click load, and load your Monsters University save game. when it loads. the second players screen will get a message saying that they have an incorrect character. IGNORE THIS. Do not hit anything or it will remove player two from the game.

Then just finish 1 mission with the first player and the achievement will unlock.

Video guide for those who are still having trouble. Notice in the bottom right corner the version number.