Dividing organization in two different organization

An organization (enterprise) consists of two departements. Theses two departements exist in foure countries. These two departement share Active Directory and some ERP systems.

Now the organization wants to divied these two departements in a way that they use two different domain and they will not share ERP systems.

I am a consultant and I have only worked with applications, platforms as solution architect. I have not worked with such kind of infrastructure projects which ERP and AD is involved.
I believe it is not so hard but I don’t know where and how do I start. I need to do some pre-studies.

What must do I do? How do I start?

I appreciate all kind of advice. Thanks.

Just create two sites in frappe bench.
Each site will be accessible by its own domain.
Both will be independent of each other.