In Active Directory Domains and Trusts => Domain Properties => Outgoing and Incoming Trusts Properties, when I click validate, an error appears:
Windows cannot find an Active Directory Domain Controller for the domain. Verify that an AD DC is available and then try again.
Both servers can ping each other's FQDN and can manage each other through Server Manager => All Servers.
Both servers are running with Windows Server 2012 Standard, connected remotely through IPSec VPN, and AD configured as a Parent and Child.