Losartan and valsartan have the same half life of 6 hours so why do some doctors say that losartan has to be taken twice a day to be effective while valsartan is considered a once a day pill?

You are correct about the half life of both drugs. However, we do use valsartan twice a day in some patients as well if dose is covered by insurance. It is not uncommon that I prescribe losartan twice a day to some patients when the blood pressure is not stable throughout 24 hours or when I try to maximize the anti-proteinuric (reduce the protein amount in the urine) effect of losartan.