To be two-faced is to have two different
personae, usually one condescending and the other highly
spiteful. The best example of such is the classic
co-worker who pays you
compliments to your face,
congratulates you heartily when you do well, and
praises you to your peers- and as soon as you turn your back, nothing but
derogatory gossip spills from their mouths, the majority of which is usually directed at your direct superior. Thus, you suspect nothing of their slander, and they can improve their status in the eyes of others.
However, once someone learns of their peer's contradictory
dual nature, their
reputation suffers greatly. Look at it this way- would you like to be friends with someone who only pays you
lip-service?