Tā shì gè liǎngmiànpài, dāngmiàn yī tào bèihòu yī tào.
He's two-faced - one way to your face, another behind your back.
Tips
usage
两面 literally means 'two faces / two sides' and works both physically (两面镜子 = both sides of a mirror) and figuratively (两面派 = a two-faced person). Context tells you which sense applies.
