Animation Retargeting with a secondary rig?

The project I am currently working on calls for a character that is having animation retargeted to it. This character requires a secondary and a primary rig. Both are separate from each other in their own scene. The primary rig will have animation retargeted on it and we want to attach the secondary rig to the primary rig inside of Unreal Editor 4. Does anyone have insight on how to accomplish this?