「期間（period）＋particle「に」＋　number +　回（かい counter suffix）」 ex)　一週間（いっしゅうかん　one week=period ） 一週間に二回　one week(period) + 「に」　＋　２(twice ) 回(times) = twice a week 　　※一週間二回 is abbreviated.(に)　In casual conversations,you can use this expression. 一か月（いっかげつ　one month= period） 一か月に二回＝twice a month 　　※一か月二回is also OK.

You usually say "週に１回" for "once a week" and "週に２回" for "twice a week". 〔Other than that, you usually use Arabic numerals, not Chinese ones, when you write Japanese sentences horizontally.〕

twice a week.

一週間二回：TWICE a week while 一週間一回：ONCE a week. Hope I could help you.