People, faces, and users all belong to tenants. Within a tenant, directories divide persons, faces, and face data. Identity synchronization can be used to copy a directory from one tenant to another but it can't be used to copy data within a tenant.
A different user in the cloud can be used as a sync partner as long as the tenant is different. The directory will be duplicated.
A different user in the cloud can be used as a sync partner but not if they both belong to same the tenant. If the tenant is the same there will be conflicts.
More complex configurations can be designed to sync data across multiple locations using combinations of cloud and local platform services.
Satellite offices or sites are configured to sync with a central server. This could be a combination of cloud and local platform services.
Local platform installations could also be configured in a daisy-chain type configuration:
Satellite offices or sites are confiured to sync with a nearest node or different account. This could be a mix of cloud and local platform installs.