wittywork,
Thanks for getting in touch! Unfortunately, our custom recurrence will currently only allow you to do one of those options (either the 1st Tuesday of every month, or the 3rd Tuesday of every month: screenshot). As a workaround, you could create two recurring events with the same name, and have one recur on the 1st Tuesday of every month and the other could recur on the 3rd Tuesday of every month.
I know that’s not ideal, but we’re currently working on enhancing the custom recurrence functionality, and would appreciate your feedback in this UserVoice Topic.
Hopefully that helps to answer your question, but let me know if I can help further. Thanks! 🙂
-Casey-