HP2 AND HP6!

Is it true that was the working title of Harry Potter and the Chamber of Secrets?
Yes. Lizo Mzimba of CBBC interviewed J.K. Rowling

What was the original working title of Chamber of Secrets?
Harry Potter and the Half-Blood Prince. I quite liked that title, unfortunately the story bore no relation whatsoever to the title by the time I'd finished.

J.K. Rowling also said, on her site:

I was delighted to see that a hard core of super-bright fans knew that the real title was once, in the long distant past, a possibility for 'Chamber of Secrets', and from that deduced that it was genuine. Certain crucial pieces of information in book six were originally planned for 'Chamber of Secrets', but very early on (first draft of Chamber) I realised that this information's proper home was book six. I have said before now that 'Chamber' holds some very important clues to the ultimate end of the series. Not as many as six, obviously, but there is a link.

Harry Potter and the Chamber of Secrets had different titles. In fact, as everyone now knows, it was once called Harry Potter and the Half Blood Prince, but then I removed a whole storyline that did not work at all. It gave too much information too early, so I pulled it out and it became a major part, but not the only part, of book six. There is no trace of that storyline left in Chamber of Secrets. People have been speculating that book six is a spinoff of book two, but it is not.