If you try to log in directly to a sub-domain Business Unit, you find that it is not possible. To access such other Business Units, you would need to use Web Services.
Trying to log in directly to a sub-domain Business Unit is not possible. Instead, a Proxy must be set up from the top-level Business Unit.
Here are the steps to achieve that:
Release: CASVCT99000-12.9-Service Catalog