It's not quite two faces in the same sense as Janus, but you may be interested in several of the mythological beings depicted in the art of the Chavin cult, a pan-Andean religious phenomenon from 800-300 BC. Much of Chavin imagery has faces meant to be viewed from different directions, likely in interaction with psychoactive drugs. The best example of this is the Stela Raimondi, which has a figure with a tall headress when upright, while the headress becomes a crocodilian mouthdown upside down.

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