"Whitey" Bulger Trial: Stephen "The Rifleman" Flemmi, reputed mob boss' ex-partner, to testify against him

(CBS/AP) BOSTON -James "Whitey" Bulger and Stephen "The Rifleman" Flemmi.

In Boston, the two names have gone together for four decades.

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Prosecutors say they were partners in crime, gangsters who together led a criminal organization that ruled Boston's underworld for more than two decades through fear, intimidation and violence.

The two men haven't seen each other in almost 20 years. On Thursday, they will come face to face in a Boston courtroom when Flemmi takes the witness stand against Bulger at his racketeering trial.

Flemmi will sit just 10 feet away from Bulger in what promises to be a tense meeting as Flemmi is asked to name Bulger as a killer, an FBI informant and the man who he watched strangle two 26-year-old women.

Bulger has already had two profanity-laced outbursts during the trial, one directed at his former protege, Kevin Weeks, and the other at a former FBI agent who admitted taking payoffs from Bulger.

Investigators say Flemmi's testimony will be the ultimate betrayal to Bulger, given their long relationship as criminal partners and friends.

"These guys were equal partners. One was not subservient to the other," said Michael Kendall, a former federal prosecutor who investigated several of Bulger's associates.

"Now, with Flemmi testifying against him, I think it's going to be like when Dracula fights Frankenstein -- the two personifications of evil at each other's throats," he said.

The two men met in the late 1960s and became partners in the 1970s.

Bulger, of South Boston, was with the Winter Hill Gang, while Flemmi had ties to the New England Mafia. Together, they built a criminal organization that made millions by controlling and extorting bookmakers, drug dealers and loan sharks.

Both became FBI informants, and by the late '70s, the two men were meeting together regularly with former FBI Agent John Connolly and providing him with information on the rival Mafia, as well as other criminals, prosecutors say.

Bulger fled Boston in late 1994 after being tipped off to an upcoming indictment by Connolly, according to testimony from Weeks. He was one of the nation's most wanted fugitives for more than 16 years, and was finally captured in Santa Monica, Calif., in 2011.

Flemmi stayed in Boston, was indicted and has been in prison ever since.

Flemmi, now 79, pleaded guilty to 10 murders in a deal with prosecutors that spared him the death penalty for killings in Oklahoma and Florida. He is now serving a life sentence.

Bulger, now 83, is accused of participating in 19 killings during the 1970s and '80s.

Bulger's lawyers have vehemently denied that Bulger was an informant and have spent much of the trial trying to refute that claim.

When Flemmi takes the stand Thursday, he is expected to testify that he and Bulger were informants. He is also expected to say he watched Bulger strangle Debra Davis, Flemmi's much-younger girlfriend, and Deborah Hussey, the daughter of Flemmi's longtime live-in companion, Marion Hussey.

"Flemmi's testimony is going to totally contradict Bulger's representation that he didn't kill women or wasn't an informant, and that's not going to sit well with Bulger," said Tom Duffy, a retired state police major who investigated Bulger.

Kendall said both men shared a "preening arrogance" that made them compatible. Both were considered intelligent, kept themselves in good physical shape and had many girlfriends.

"There is a certain poetic justice that Stevie is ratting out Whitey," Kendall said. "They know each other's secrets better than anybody else and they are equal to each other in terms of their criminal behavior and depravity."

Complete coverage of the Whitey Bulger case on Crimesider