Tā shì ge shuāngmiàn rén, dāngmiàn yī tào bèihòu yī tào.
He's two-faced - one way to your face, another behind your back.
Tips
usage
双面 literally means 'double-sided.' It can describe physical objects (双面胶 = double-sided tape) or metaphorically describe a deceptive person (双面人 = two-faced person).
