How do you describe something that takes place every two weeks?  How about something that takes place twice per week?

If you use bi-weekly for either situation, don’t.  If you use bi-weekly for both, give yourself a slap.  You’re not wrong, but there are better terms available to you that won’t cause confusion:

• ‘Fortnightly’ – something that happens every two weeks
• ‘Twice-weekly’, ’semi-weekly’ – for something that happens twice per week.

[Post inspired by the 'Fiver Letters' section of the 18th August 2009 edition of The Fiver, The Guardian's daily email newsletter about football and a surprisingly rich source of linguistic pedantry.]