Adding an additional SAN certificate (Subject Alternative Name) with SSL Certificate purchase, can secure multiple domains on single server.
If you want to secure two domains on single server, then you should buy a 1 certificate with adding a 1 SAN certificate during purchase.
Below table shows the number of domains on single server with requirement of SSL Certificates:
2 domains = 1 Certificate + 1 SAN
3 domains = 1 certificate + 2 SANs
: :
: :
N domains = 1 Certificate + (n-1) SANs
Generally, Major certificate Authorities allows you to add additional SAN up to 24. It may vary by Certificate Authority.
###Selection of Certificate:###
There are three types of SSL Certificates DV (domain validated), OV (organization validated) and EV (extended validation) certificates. You should buy OV or EV certificate, because it has better SAN option then the DV certificates.
