Would it make sense to allow the user to choose receiving email notifications to both their primary email and their secondary one? Or should I just limit the user to selecting just one?
If email notifications are enabled, sending them only to the primary email is a sufficient option. It is more concise and requires less maintenance.
The secondary email most common use is for access recovery only. Sending notifications that are not relevant to the access control to the secondary email seems like an overcomplication. If a user wants a different email for the regular notifications, he or she can make it the primary one at any time.