Here's what Fredric Effects says about the DuoFace:

The Fredric Effects DuoFace fuzz is based on two versions of the classic Fuzz Face* circuit, combined into a single pedal for maximum versatility. Uniquely, the DuoFace incorporates both the silicon transistor version and the germanium transistor version of the FF circuit through the use of a voltage converter. The circuits are distinct and separate and have independent bias controls.

*Fuzz Face is a trademark of Dunlop Manufacturing, Inc.