“Biannual”, “bimonthly” and “biweekly” must be some of the most useless words in the English language. This is because each word has two different meanings.
They can either mean “occurring once in two years/months/weeks” or “occurring twice in one year/month/week”.
If you don’t believe me look them up in a dictionary. It’ll say the same thing, but it’ll make no apologies.
But it should, because if you say, for example, “The above payments should be made bimonthly” nobody knows whether you mean these payments should be made twice a month or every two months. Or if you say “The organisation publishes a biweekly newsletter”, nobody knows whether you mean the newsletter comes out twice a week or every two weeks. (By the way, every two weeks could mean bimonthly – i.e. twice a month. So it is possible for a biweekly magazine to also be a bimonthly. Which is silly.)
So I recommend you avoid using these words.
Here are some more sensible alternatives:
occurring once in two years: every two years / biennial
occurring twice in one year: twice a year / semi-annual
occurring once in two months: every two months / every other month
occurring twice in one month: twice a month / semimonthly / every two weeks (UK only: once a fortnight / every fortnight / fortnightly)
occurring once in two weeks: every two weeks / every other week / twice a month / semimonthly (UK only: once a fortnight / every fortnight / fortnightly)
occurring twice in one week: twice a week / semiweekly