For me, the word "bimonthly" has always been confusing whether it means twice a month or once every two months. Looking up the word in some dictionaries today, I noticed that all the dictionaries I checked it up define the word as something like "every two months OR twice a month". Below are some examples.
Collins Dictionary https://www.collinsdictionary.com/
1. every two months
2. (often avoided because of confusion with sense 1) twice a month; semimonthly
1. occurring every two months
2. occurring twice a month : semimonthly
Oxford Learner's Dictionaries https://www.oxfordlearnersdictionaries.com/
produced or happening every two months or twice each month
Longman Dictionary https://www.ldoceonline.com/
appearing or happening every two months or twice each month
Since the "every two months" definition always comes first in all those dictionaries, I assume that it is the commonly-understood meaning by native speakers in general, but how can you be so certain that it really means every two months?