WordWeb Online

1. Having two faces--one looking to the future and one to the past
   - two-faced
    
2. Marked by deliberate deceptiveness especially by pretending one set of feelings and acting under the influence of another
   "she was a Janus-faced scheming little thing";
   - ambidextrous, deceitful, double-dealing, duplicitous, two-faced, double-faced, double-tongued
    
3. Having or concerned with polarities or contrasts
   "a Janus-faced view of history"; "a Janus-faced policy"

See also: bipolar, dishonest, dishonorable [US], dishonourable [Brit, Cdn], faced