[OJFord]

...in the US.

I only have Collins' concise to hand, which gives the definitions I have and doesn't even mention (not even proscribed or US etc.) other uses. And Collins is pretty permissive.

And no they're not. 'Biennial' events occur once per two year period; 'biannual' events occur four times per two year (twice per year) period.

For example, in horticulture 'a biennial [plant]' is one that has a single flowering/reproduction cycle over two years. (Cf. 'annuals', 'perennials'.)