Frustratingly enough, it's the other way around. Biweekly is every two weeks, semi weekly is two times a week.

I remember it like this:

• bicycles are two circles, biweekly is two weeks
• semicircles are half a circle, semiweekly is half a week

But yes, people use the words interchangeably so often that it's faster just to avoid the problem altogether and just say "every two weeks".