Ted here from Shopify, thanks for posting your query!
Great question! An account can have up to 10 domains (11 if you include the .myshopify URL), all with free SSL certificates provided by Shopify. Shopify Plus has no limit.
So you can add 10 domains to a store and Shopify will provide SSL certificates for free to each domain. This includes the "www" subdomain, or other subdomains, but does not include the "myshopify.com" domain.
If you're asking if it's possible to set up two different websites/online stores with the one account, it wouldn't be possible. You can set up URL redirections if you're hoping to point the domain/url to a specific page, however, it wouldn't be possible to have two separate website layouts with the one account. If this is not what you mean, please let me know with an example and I'll take a closer look.
If there is anything else I can help you with, please let me know, I'm happy to help!
Sorry for the delay in my response, I never received this notification and I missed your query. Would you be able to show me an example of what you mean by "update with other content"? Are you hoping to have the domain point to a non Shopify store