CIC allows you to sponsor a common-law partner while still being married to someone else.
http://www.cic.gc.ca/english/resources/manuals/op/op02-eng.pdf
5.38. What happens if the common-law partner (principal applicant) is married to another person?
Persons who are married to third parties may be considered common-law partners provided their
marriage has broken down and they have lived separate and apart from the spouse for long
enough to establish a common-law relationship – at least one year