We build Subordinate CAs not really for the purpose of redundancy, like with many other kinds of servers, but because there are specific tasks that you may want to perform on a subordinate CA rather than a root CA. If you issue a lot of certificates, or different kinds of certificates, you may want to differentiate between CA servers when issuing. Perhaps you want machine certificates that are used for IPsec to be issued from IPSEC-CA, but the SSL website certificates that you issue should show as being issued from WEB-CA. Rather than building out two independent Root CAs which both have top-level rights, you should consider creating a single Root CA, maybe called ROOT-CA, and placing these two CA servers in a subordinate role, "under" the Root CA in the chain. This can also be useful for geographically dispersed networks, having Subordinate CA servers dedicated to assigning certificates for different offices or regions.
As we discussed...