Peter Duniho wrote:

American Heritage
Merriam-Webster

Maybe, but to me, the only authoritative source is the Oxford English
Dictionary (the big one), which states:

biennial (Origin: Latin biennium, space of two years)
1. Existing or lasting for two years; changed every two years.
2. Recurring, happening, or taking place once in every two years.

biannual
Used as = Half-yearly.

with no alternatives.

Interesting are the references to the first occurences:
biennial: 1621
biannual: 1877


Stefan