Thank you for contacting Yubico Support.
Thank you John Galt, but your guidance was NOT correct. According to YUBICO, I do not need a primary and backup security key for myself (using my Apple ID) and a separate primary and backup key for my wife. Yubico tech says that I may have a primary for myself, and a primary for my wife (she has her own Apple ID, and one BU Yubico will suffice and work for both of us. The YUBICO response follows: In your case specifically, the best arrangement would be to each have your own Primary Key, and the third Key can be the second (spare) Key can be for both of your accounts. We would recommend keeping this Key in a safe sedentary location. I hope this clarifies things for you.
Please let me know if you would have any further questions, I will go ahead and close this ticket but don't worry if you would reply back the ticket will open itself and I will happily assist you further. Have a lovely day!
Best,
Gabe | Customer Support Specialist at Yubico