Bimonthly means every two months OR twice a month.
Therefore biweekly and bimonthly COULD mean the exact same thing. OR not. Why in the world would you define an adjective for frequency so loosely in terms of frequency??? Its not very helpful, because now that I know this I will always second guess how its being used. I guess you may second guess this now too. Sorry! Just say twice a week or twice a month instead. Then everybody is clear.