You can, but usually when talking about redundancy you do it at the subordinate level and just issue the same templates to both online CAs - they will flip-flop for issuance and if one is not available the other will be, even if renewing an autoenrollment cert from that was originally issued from the other CA.
The offline root is there for security reasons, not for high availability. It is easy enough to do a windows backup with system state to properly recover the root if need be, or if you install it in a VM to just backup the image to an external hard drive that you keep locked up/offsited.
When creating backups you should backup the private key of the CA and keep it super-secured locked up somewhere. In a disaster situation you can then restore the failed CA's private key onto the production CA and use the certutil -sign command to re-sign the CRLs, etc. while you are recovering the CA. This is also why it is a good idea to issue your CRLs via script (certutil -crl) via script at the half-life of the CRL to allow for overlap so you have time to recover from issues. More info here:
http://www.pki101.com/BackupCA.html