I just faced a similar situation with a three-camera shoot and tried two different workarounds:
1. Exported just the audio tracks from each clip, then combined those tracks into a single separate track in an external audio-editing application; and
2. Exported just the audio tracks, then made them into a single multicam clip within FCPX.
Both approaches worked, but I preferred and ended up using Option 1.
–JN
Joel Nisson