Is it possible to create a new windows root domain named subdomain.domain.local in the same forest where the root domain domain.local already exist? Please note I am not saying to add a child domain to domain.local, but to create another root domain in the same forest.
An Active Directory forest cannot have two root domains.
From a DNS perspective, you can certainly delegate to a separate root at that name, if that's what you're looking to achieve. You can also set up a cross-forest trust between the separate roots, if that's what you're aiming for. But, a single forest only has one root domain.
Can you give some more details about what your goal is? We may be able to provide some alternative recommendations.