There was only one person formally recognized as such (Tsutomu Yamaguchi), but there are indications that there were many more, perhaps over a hundred people, who in the aftermath of the Hiroshima attack took refuge in Nagasaki and survived both attacks (I doubt we have any good estimate of how many people survived Hiroshima but died at Nagasaki; the records are just not that great). If you Google Yamaguchi's name you'll find many articles about this.

Trumbull, Robert. Nine who survived Hiroshima and Nagasaki: Personal experiences of nine men who lived through the atomic bombings. New York: Dutton, 1957: is probably the only primary accounts of such persons published in English, though it had been out of print for long (so check your local/ university library). The book has recently translated in Japanese in 2010 so that I can come across the bibliographical data in question.

Documentary Film on such double-bombed victims in 2006 (Now gone their official site in Japanese), featuring Tsutomu Yamaguchi, at least includes the interview with six more such victims, and one of them, Kuniyoshi Sato, is also mentioned in the oral testimony film found in Hiroshima Peace Memorial Museum's data base.

While the official statistics of double-bombed victims seem to be lacking and often confusing (normally only one bombed place was to be recorded even if it was such a case), NHK (Japanese broadcasting channel) confirmed in 2015 with the Nagasaki City that there were still 18 such victims alive after 70 years after the bombings (source in Japanese).