SSL Certificates purchased through us can be activated for the domain names that are neither registered, nor hosted with us. It is possible to have Certificates activated for the same common name with different providers. In case your domain name is not hosted with us, please make sure your web host allow installation of third party SSL Certificates.
SSL certificate installation requires a dedicated IP address to be assigned to your domain name. Each certificate requires a separate dedicated IP in case SNI technology is not available with your server type. There should not be any contradiction/collisions between Certificates purchased from different providers on the server. However, your server will most likely be able to address to only one certificate.