1) Better half 2) He took a second 3) Lawbreaker with two partners 4) Married person 5) Mate 6) Partner 7) Person having one too many 8) Spouse 
• (n.) One who is guilty of bigamy.
Big'a·mist noun [ Confer Digamist .] One who is guilty of bigamy. Ayliffe. 
[n] - someone who marries one person while already legally married to another
bigamist 1. A person who commits bigamy or someone who is married illegally to two people simultaneously.2. Said to be someone who demonstrates that 'two rites make a wrong' and an action by someone who is participating in 'double jeopardy'.
noun someone who marries one person while already legally married to another
a person who commits bigamy.