It's no perfect solution, since crontab works in months, not in weeks, but the pattern I'd suggest is:

0 3 */14 * *, which executes a job on every 14th-day (at 3 AM) (i.e. 14. and 28.), which is close to bi-weekly, but since most months are 30 or 31 days long, you actually have: An execution on the 14th, 2 weeks pass, another execution, 2 weeks + 2-3 days pass, another execution, then exactly 2 weeks pass, etc.

If it has to be exactly 14 days apart, it could be a bit more tricky.