We have three CAS servers in our environment. Each one has its own SAN certificate. I was reading somewhere you can purchase one and install it, then export it out with the private key and install it on another CAS servers. Most of the common names like webmail.domain.com, autodiscover.domain.com etc...are the same for all CAS servers. The only ones that would be different is the CAS FQDN and SMTP name so you would just have to make sure they were all included in the SAN.
Thing that concerns me though is when you generate the first SAN on the first CAS, it creates a private key that you can see when you issue a get-exchangecertificate command. Later, when you get your certificate from the vendor, you can import it in and the private key is there to match the certificate. On any other CAS servers, the private key is not there so will there be a problem, or does exporting the private key on the first CAS take care of this?
These SAN certificates are expensive and we have several so reusing the same one would be a big savings.