I wouldn’t recommend using port forwarding. Active Directory relies heavily on consistent IP addresses and name resolution for its members, you’d run into similar problems as with multi-homed servers.
A setup with two domain controllers bound to their respective LAN IPs might work, but it’s not supported, as we don’t yet support multi-site AD configurations.
If you’re aiming to add a second file server, this How-To shares some useful tricks, though keep in mind you’re entering unsupported territory:
As alternative, as we’re developing a File Server AD member role, you can also wait for its release instead of using two DCs.