Typically, a "shared certificate" will be a wildcard certificate installed for a domain. For instance, some if there were a shared certificate available for *.dreamhosters.com, we could all use ssl (https) for our dreamhosters subdomains. If we set those domains as mirrored domains, we could mirror all or some of the content at our primary domains.
What you could do is buy a wildcard certificate for *.yoursecuredomain.tld. You could then use one subdomain per client site - for example, client1.yoursecuredomain.tld might be used for client1.tld.
You could alternately and more cheaply just use a single domain with mapped subdirectories. An example of this would be yoursecuredomain.tld/client1.
In all of these cases, you will have to direct your clients "outside" of their assigned domains to some shared domain unless you're going to have you clients use the subdomains in the first place.
BTW, the other problem is that wildcard certificates are much more expensive. The last time I looked to buy one they were $1000 a year and up but I just searched and found one from a super-discount vendor for around $200 a year. This same vendor has regular cheapo certs for $20 a year.
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