New forest using subdomain of existing forest namespace

I've never encountered this before, but I'm hoping someone may have. All new forests I have ever created even for resource purposes etc have always had their own dns namespace i.e.

Forest 1 = domainname1.com
Forest 2 = domainname2.com

Which makes things nice and simple, now I have a need to stand up a new resource domain, but try and keep an extended namespace via a subdomain i.e.

Forest 1 / root domain = domainname1.com
Forest 1 / child domain = something.domainname1.com
Forest 2 / root domain = somethingelse.domainame1.com
Forest 2 / child domain = thisdomain.somethingelse.domainname1.com

So my thoughts are delegate that zone down to the new servers for the new forest. In theory because they're distinct namespaces with no overlap at that point things should work as from a trust point of view everything DNS wise could be resolved. It would then need an exclusion on the original forest side to exclude somethingelse.domainame1.com from that side as implicit for authentication

What are peoples thoughts on this? I'd prefer to have a completely new DNS domain for the new forest and can't see much in the way of google for this one

Thanks in advance

Technet's article on Domains and Forests is pretty good and seems to cover what you're trying to do. Basically your plan carves up the DNS Namespace into two trees under the one forest, I think that's what you're trying to do?

Keep in mind you generally don't want to use the TLD for anything internal because it makes inside / outside issues for DNS more likely.

Yeah, don't use a TLD like companyname.com, but there's nothing wrong with registering companylan.com and not publishing any DNS records for it, and then using that internally.

Normally you'd just use lan.companyname.com or ad.companyname.com or whatever though.

So to clarify this is reusing a subdomain of the same TLD in a new forest. There is already a TLD used in the root of the current forest, as there is no use for it externally other that a couple of controlled websites. So the normal split DNS issues aren't a problem here.

Is there any reason it has to be a new forest?

For security/strong isolation really

Just out of curiosity, why is this being done this way? Apart from the fact that it makes for long URLs, it is going to be error-prone and will probably cause all sorts of little issues in the future. Much cleaner and simpler as you said, 2 completely different zones (users.local and resources.local).