Hi Melody and welcome to the forums!
That’s a good question and I’m surprised it doesn’t come up more often.
You’re not missing anything at all. For now, the custom monthly recurrence only accepts a single condition (e.g. second Sunday of each month) rather than multiple conditions (second and fourth Sunday of each month).
However, a different way to go would be the custom weekly recurrence. For example, you could set the event to recur on Sundays every other. Then again, that might place some instances on weeks that are not the second or fourth Sunday–give it a try though and you can always delete specific instances from a recurring series.
Will that work for you? Please let me know. 🙂
Cheers!
Geoff