In my experience here and out, i see various IT scenario. More or Less have a simple infrastructure with single domain controller.
Have a single point of Failure for a medium enterprise is not always a good idea, also if they have a solid Backup system. I suggest always 2 Domain controller, in replica, in separate host, separated location is better.
I can help you to consolidate your infrastructure and migrate/integrate a new domain controller in your infrastructure totally redudant.
Have a solid infrastrucuture can help you to not have a service downtime, and more service added.
Leave that my experience can help you in work days