Lmao I'd interpret that as every two weeks. Semi meaning "almost", so "semiweekly" would mean almost weekly, hence, every two weeks. I guess you could think "almost" the other way but I feel like semi is usually used in a way that is "quite but not as good", twice a week would be more than once a week so I semi would have to be every two weeks in my mind.
I'm not being cheeky. That's how I perceive the thing, but I'm not a native English speaker and french has a way to distinguish between both cases without using semi(which is confusing too imo).
Bimensuel => 2 times a month
Bimestriel => every two months