Eastward

You'll unlock this achievement when you first attack an enemy with Sam's Energy Bubble attack using with pointed toward your enemy to paralyze, then use either / to swap for John and use for his frying pan swing to defeat them. I think you can do this is in Chapter 1 when you can control both John and Sam as they go through the underground tunnel leading up to the Mayor's Estate in Potcrock Isle. I say this only because I didn't unlock this achievement until the following chapter.