Biennial and biannual are two terms very often confused. Based on the usage of the pattern of using bi-" in other contexts, one would use these two terms interchangeably.
Biannual means something occurs twice every year, or two times within a 365-day period. A biannual session is one take takes place twice a year.
Biennial means that something either occurs once every two years or exists or last for two years. The way to remember this is to think of a famous film show in Venice, called "Biennale di Venezia". It is held every two years.
As you see, there words are far from being interchangeable.
