This is how you get the 'Heads I Win, Tails You Lose' achievement in Lego Batman for the XBox 360.

Details: Defeat 10 police officers and 10 goons with Two-Face in a level.

This one seems a little misleading b/c it's worded in a way that makes you think you need to do it in the same level, which is not the case.
You need to have two face unlocked from Chapter 1: Area 4 - Villain.
In this level, it's likely you killed the 10 cops already, so head to any level with goons and get at least 10 kills (maybe less) for this one.