The truth is that you probably will get a rejection, because you need two interviewers to say "Yes" for you to pass to the next round. And if you second interview was really-really bad as you've described it, then you need to accept the rejection and try again being better prepared next time, or basically continue interview with other firms and get other offers fro their. There are many other opportunities.
Regarding writing to HR, I would not really recommend you to do this, because it might bring you into very negative light for future.
That's basically it.
Wish you good luck,
and if you need any support in order to get offers from other firms, feel free to drop me a line,