Bigamy is having a second wife at the same time as a first wife.
ex. After hanging around with a lot of really nice people in Utah, he became a bigamist.
Digamy is having a second wife after having lost a first wife (through death or divorce).
ex. After a lot of court dates, awkward dates, and finally good dates, he became a digamist.
I've heard the term "serial monogamy" used to indicate digamy+, but I think I like digamy better. As a usage, obviously. Then again, trigamy appears less clear so maybe it won't work.