You can create multiple copies of your YubiKey using the instructions below. This can be useful so that you have a backup, in case the original YubiKey is lost.
To do this:
Program the first YubiKey for HMAC-SHA1 Challenge Response
Make a record of the unique "secret" that is generated
Program the second YubiKey in the same way, except manually enter in the same secret that you used for the first YubiKey
Virtual Hardware Keys
It is also possible to take that same secret and create a virtual hardware key.
A virtual hardware key is a software only implementation of the hardware token. As long as it it is stored in a secure location, it can be an effective backup for your physical YubiKey.
It is also required in order to use a YubiKey with AutoFill on iPhone and iPad.