Here is another pair of words that looks the same but each has its own meaning. 
- bienniel – once every two years
- biannual – twice a year
If you want to say that your organisation conducts financial reports twice a year, then you’ll say: “…biannual financial report”
On the other hand, if you report only once every two years, then you say: “… bienniel financial report” – err, is there anything to hide? 
Why not report annually?
Example:
“We can’t afford to have a biannual event, so let’s just have a bienniel one.”