Just say what you mean:
“every 2 weeks,” or
“twice a week.”
Pick the one you mean.
Because unfortunately “biweekly” can mean either of those things, which kinda makes the word useless.
People in my area of the English speaking world have the horrible habit of saying things like “let’s meet at the bar back of 8” or “I’ll call you back of 6”. I have never got a satisfactory answer if this is “just before ópm” or “just past 6pm”, or even “near the end of 6, so almost 7pm” Every person seems to interpret it differently, it’s wild. And profoundly unhelpful.
I only use it when referring to bi(sexual)weekly.
The team I was on at work number of years ago had to hash this exact thing out because we had a “biweekly” status and was confusing people. We ultimately landed on not using the term.
Bimonthly and biannually have the same problem.
Yes, because I am a simpleton and Fortnightly and ‘twice a week’ already exists.
Get out of my head
There are not two definitions. The prefix (bi) modifies the root (week). The suffix (ly) modifies the combination (biweek). Biweekly will always mean “once every two weeks” due to the order in which English dictates that the modifiers operate.
There are two definitions. The world of English disagrees with you.
There are two definitions, according to the major dictionaries I can view (OED requires a subscription):
https://www.merriam-webster.com/dictionary/biweekly https://dictionary.cambridge.org/dictionary/english/biweekly https://en.wiktionary.org/wiki/biweekly https://www.dictionary.com/browse/biweekly
So, the usage of biweekly as a synonym for semi-weekly is sufficiently high enough to land it in the dictionary, alongside a definition of every-two-weeks, and make biweekly an ambiguous and confusing term.