Did a quick search and didn't find anything obvious, so I'll just post this quickly for today, then do some digging when I have a moment...
In short, when using the JSON API plugin with Co-author Plus, the default output from the JSON API doesn't pick up on additional authors for a post.
Would be great if the two plugins worked together, given how freakin' awesome they both are. :)
I'll poke around for a quick fix, but happy to hear suggestions/comments too.