|Do you understand the difference between a domain and a forest?|
If you pursue the plan at hand you will end up with the same forest [namespace].
That is not a good plan.
I have to ask:
Why do you need two separate networks?
More specifically what do you mean by two separate networks?
Reason I ask is you are doubling your administration and overhead.
Why not have the same forest but have two different domains?
Divide the network via subnetting or vlans.
Lots of options, have you considered them?
BTW having a single DC for a forest is not recommended. MS even says so. If you really want to embark on this plan you need two dcs in each.
Otherwise you keep the same forest, have two physical DCs, and have two domains in the same forest.
It really comes down to if one of these "networks" is going to have a Internet Domain name and you are publishing a web page.
If not, one forest is the way to go.