Adding additional audio to a synced clip and having that sync via audio in Final Cut Pro X In FCPX I can select a video and audio clip and do "Synchronize Clips" using the "Use audio for synchronization" option.

Now I have a new clip in the browser on which I can say "Open Clip" and then see the timeline with the audio synced to the video.

If I want to add another audio track into this synced clip, I want a way for the additional audio to be automatically synced too. 

I can't seem to find a way to do this - is it even possible?
This is a long shot as I did not try it. 

Try to use a Multicam clip instead of a Synced clip. 
I know that it is possible to add another audio line to a Multicam clip and sync it inside the Multicam clip.