Client has a SAN certificate for the following domains:
Certificate is already installed for domain1.tld. How to setup for second domain
Officially SAN SSL certificates are not listed in the supported features for Plesk.
SAN certificate is valid for several domains. Just install it on every domain you need on your own risk:
Home > Subscriptions > domain2.tld > Websites & Domains > Secure Your Sites
Assign certificate to domain:
Websites & Domains > Hosting settings > Security > Certificate