Rahman did make one successful title defense. After Vitali Klitschko canceled a defense against Rahman, Rahman defeated Monte Barrett for the interim WBC title in August of 2005. When Klitschko retired, Rahman became the regular WBC champion and successfully defended the title with a draw against James Toney.
Shannon Briggs can be considered a two-time champion if you consider his win against Geogre Foreman as being for the linear championship. He lost the linear title (the real title) to Lennox Lewis in his next fight.
I think Briggs counts. The linear title means just as much as an alphabet belt. I don't think Rahman's draw with Toney was a successful defense though. He didn't successfully defend it, he just didn't lose it. It was a draw.
A draw does constitute a successful defense. If a champion enters the ring as the champion and leaves as the champion, he has successfully defended the title. Everybody says Bernard Hopkins made 20 successful title defenses. One of those defenses was a no contest against Robert Allen.