Not to join the posting police, but I think you mean biennial (once every two years, not twice a year).
Yes, I remember discussing the differences with someone else a while ago. It’s Bi versus Semi.
To solve the issue once and for all I chose to Google it.
Below is the best description I could find. LOL
“Bi-yearly would be every two years. (Think— bisexual isn’t someone who’s attracted to half a gender, its someone who’s attracted to two genders)”