As I understand it, the existing Portal DR option requires: "The primary and standby environments must be duplicated. Each data center must have the same number of machines in the ArcGIS Enterprise deployment, and the machine host names must be the same." Disaster recovery and replication—Portal for ArcGIS (10.5) | ArcGIS Enterprise
I suggest the DR pattern should be possible with different machine host names, rather than them needing to be the same. As far as I am aware it is not possible to have machines with the same host name on a single domain. My employer is fairly typical in that it has more than one data centre in separate geographic locations but they are all connected to the same domain. I can't currently deploy the DR pattern so can't provide a Portal DR solution As DR is important to the business, Portal will not be used as widely as it would be, should we be able to put a DR solution in place.