A more thorough explanation:

Janus-faced (jay-nəs fayst), adj. Having two contrasting or contradictory aspects; two-faced.

• A politician who promises to lower taxes but also promises to increase spending is making a Janus-faced proposal.
• A friend who talks behind your back but acts friendly to your face is being Janus-faced.

The examples illustrate the definition of Janus-faced by showing situations where someone or something has two contrasting or contradictory aspects. In the first example, the politician is promising two things that cannot both be true at the same time. In the second example, the friend is being friendly and unfriendly at the same time, which is contradictory behavior.