There’s really nothing to configure - it just works.
You might have a check on name resolution, since Domains are often tied to DNS. The clients on both domains should resolve the (fully-qualified) names to your root server and Relays.
Are you using domain credentials to log on to the console? Will you want to add credentials from your second domain, or just use console credentials from one domain to manage them both? The “Use Windows Credentials” checkbox at logon is based on Kerberos, so only operators from the same Forest as the server can use that; buy if you add a second LDAP source you could use domain username/password entries.from both domains.