Sorry network-marvels but I have to bust yer chops.
Your Q&A is very simple, and many of my original concerns on this issue are of concern.
Since I am a system admin., I don't want to wait for new Yubikeys to be shipped until I can check email/login to websites, etc.
Additional details/background on past discussions of this subject:
Dual usage Yubikey
viewtopic.php?f=6&t=287&p=1392#p1392Winchester Password System should be mandatory for Yubikeys.
viewtopic.php?f=6&t=311&p=1452#p1452Lastpass, now someone there obviously listened to our concerns and they offer a paid pro account where a person can have up to 5 yubikeys registered for use with a single account ID. (it's my opinion it has a been well designed for privacy and security)
Personally, I would tell people who ask "what happens if" that if they have any concerns to buy Yubikeys in pairs, and only use them with programs/websites that allows for 2 or more Yubikeys to be registered (per account) to grant access.