This is in fact very easy and you do not need to associate certificates with accounts manually.
Indeed, you just need to have a look at the e-mail address you give to Thawte ( or any other certification authority of your choice ) when you request the certificate.
This is the address that will be associated with the certificate.
Then, once you have your first certificate, provide an additional address to your CA and request another one... All the information that you need to do that may be found in the article or on Thawte's FAQ pages — see the links above.
Mail will automatically locate the various certificates in your Keychain and use the appropriate one with each e-mail address.
Let me know if this helps !