This, but somehow with biweekly and biannually there's long been some corruption where everyone thinks semi- doesn't exist as a prefix.
Actually, I think historically semiannual and semiweekly is a late introduction to avoid the original confusion. The real kicker is how hard is it to say we meet every two weeks instead of we meet biweekly or bimonthly? At least for the two weeks one if you want to be cool you can say "we meet once a fortnight". I'm pretty sure if it's not 1776 and you're not talking about sex or chemistry there's not much reason to use a bi- prefix.