The validation process depends on the type of SSL Certificate. The only common feature for the multi-domain certificates we provide is that each domain in such a certificate should be validated before issuance.
The domain-validated certificates will give you an opportunity to select the most suitable verification method for each domain: approver email or alternative HTTP-based validation method. You are not obliged to use the same validation method for all domains inside multi-domain certificate and are free to choose the one that suits you most.
When you are considering the domain with organization or extended validation, it is necessary to remember that all the domains inside the certificate should belong to the same company.
The organization-validated certificates come with a site seal which can display only one company name; the same way the green bar which is enabled by the EV certificates (the ones with extended validation) can show just one company name as well. That is why domains belonging to different organizations require separate OV/EV certificates.