On examining the job (launched from Pig), one sees that FileOutputCommitter is being called twice (as expected), once for each of the Storers. The problem is that both times, "mapred.output.dir" is set to the same temp-directory (corresponding to one of the stores.)
This looks like a Pig bug to me. PigOutputCommitter should be setting the right mapred.output.dir before invoking setupJob() on the underlying committer. (Will raise bug.)
I'll put up an HCat patch with the workaround, in a few minutes.