Unfortunately, our system is limited to 30, 45, 60 day frequencies. However, you can subscribe to the same product 2x and set each charge dates 2 weeks apart at a 30 day frequency which would an order to process every 2 weeks.
For example, you can create 2 active subscriptions, and schedule the charge dates 2 weeks apart with 30 day frequencies each. You can create 1 subscription and schedule it for Sept 2, after it has been processed the next charge would be scheduled on Sept 30 (30 day frequency). Meanwhile, the second active subscription schedule it for Sept 16, after it has been processed the next charge should be scheduled on Oct 14 (30 day frequency).