For me. biennial can only mean every other year and never twice a year. If I really wanted somebody to understand what I was saying I would say: twice a year/week or every other year/week (every two years). The way that bi- and semi- work in Latin is as two-X and half-X, where X is some thing like a year or a foot, and not as twice-in-an-X.

Ceci n'est pas un seing.