Make sure you have The City DLC purchased, or that it's on rotation for free trial, and activated for the game. Choose the Spy as your character. Add two local players that you can control, and set both of them also as the Spy character.
When the game starts, you need to choose a place to start that has another character, and the only available choice is the City. So choose that for all three.
Then roll for and move your character. Next, roll for and move the second player. With the third player, use the Shadow ability to move them straight to the second player. Then it's your turn again, so choose to Shadow either of the other two, now both on the same space. You'll move to the same space with them.
At this point you can't choose to encounter the player you Shadowed, but you can encounter the other one on that space. So do that, and the achievement will unlock!
Should take about 2 minutes.