You won't want to use "bi-weekly" as it can mean two things: twice-a-week, or every-other week. I don't think you need anything like this since your frequency column makes it very flexible. The term that would be correct is "fort-nightly", although I wouldn't use that either.

You can make it 'week' and put 2 in the frequency. I have gas set as 3 weeks because that's usually how it ends up being for me.