Schmidt's Piggyback Connectors can be used to wire dynamo headlights in pairs or to wire additional accessories to your dynamo hub — such as the Busch & Müller USB Werks. The piggyback connector attaches to the SON hub the same way the standard 4.8mm Female Connector would, but it also has an additional male tab to which a second female connector may be attached (shown in the second photo at left).
Relevant Dynamo Blog Posts: Dynamo Lighting or Charging Devices w/Dynamo Power