Can't create events occurring every other week

I suspect that I'll need this functionality, so leaving a note for myself or anybody else who decides they need to hit this -- I don't think it'll be too involved. The instance creation logic is in RecurringEventsFieldTypeInterface::calculateInstances(), implemented in the Drupal\recurring_events\Plugin\Field\FieldType classes. Likely just need to add the integer field (like suggested by @the_glitch) and then use that to modify the generated instances.

Alternatively, if folks want to implement this themselves, there is hook_recurring_events_event_instances_pre_create_alter(). If you add the integer field yourself, you could pull the value in this hook to filter out the instances before they are created.