It took me a while to figure this one out (with help from your title). Still, it's not clear that there's another fish in the mouth - it simply looks like this fish has a forked tongue (which would be an interesting find, actually). I would have liked to see if framed a little better to see that other fish in the mouth, and maybe pull back a little so we can see more of the larger fish (more of its eye, anyway) Good catch!