Biweekly and
bimonthly are variously used to mean "twice a week (or month)" or "once every two weeks (or months)." Because the meanings can be confusing, you're encouraged to avoid them unless the context makes it very clear which definition you're using. Another option is to use
semiweekly or
semimonthly when you want to indicate that something happens twice a week (or month). The best approach may also be the simplest one: just say exactly what you mean.