Two Faced

You can get this Achievement in Chapter 1: Lockdown. Don’t worry if you missed it, you can replay any Chapter in Thief

You need to collect both Masks, the fake (Jewelled Mask) and the real one (Lyegrove’s Jewelled Mask) to get this Achievement. The real one is the the Safe (Code 739) behind the Painting in the Cellar. The fake on is on display in the 2nd Drawler from the right on the ground floor (for the US its the first floor ^^)