Hello ChipB,
Hosting accounts on shared servers can only have one SSL per account. This is due to the fact that SSLs need dedicated IP addresses and the cpanl software that runs on the servers only allows a single IP to be associated with an account.
The only way to have multiple SSLs on a shared server would be to have different hosting accounts for each domain that needed an SSL.
An alternative is to upgrade to a VPS, where you can assign each domain its own cpanel account. This would allow each domain to have its own dedicated IP and SSL as well.
With a VPS, each subdomain can be moved to its own cpanel and then you can simply purchase an SSL for each.
Here is where you can find more information on how to obtain an SSL certificate for each cpanel account.
If you have any further questions, please contact technical support or leave a comment at the bottom of the page.
Regards,
Arnel C.