This is one is mainly for partners and CSP as they are responsible of managing different Office 365 tenant.

It is now possible to switch between different Office 365 tenant directly from the Office 365 Administration portal.

To be able to use this feature you must use the new admin experience  and have the other tenants registered with your organization (Partner relationship).

Once the feature is available for your partner tenant you will see the directory switcher top left of the home page of the Office 365 administration portal

By hitting this switcher button, you will get the list of registered tenant you are managing; you can even search by name if you have a lot of managed tenant