You can already do this, but your second domain will need to purchase a separate business subscription; websites can’t share the same plan outside of Enterprise.
To set this up, all you need to do is register the new domain, add the new domain to Cloudflare, and make sure the DNS has A records for @ (the root domain, eg example.com) and for www. These DNS records would point to your existing site’s IP address.
If f that is subdomain.mydomain.com it would require a subdomain configuration which is an enterprise only feature today. Or you would perform a NS delegation for that host name to another nameserver/ service outside Cloudflare.