In what circumstances will a defendant be allowed to call co-defendants as witnesses?

In People v. Hannon, 50 Misc.2d 297, 270 N.Y.S.2d 327 defendant and two others were jointly charged with conspiracy, the facts were such that a co-defendant indicated a desire to call other co-defendants, have them sworn as witnesses and despite objections of their counsel and notwithstanding the possibility or even the probability that if sworn they would invoke their constitutional immunity from self-incrimination, attempt to require their testimony. It is equally clear from the reading of the case that the purpose of calling the co-defendants was to place the criminal responsibility for the acts charged solely upon them and to exculpate himself from criminal responsibility. While in the Hannon case the trial court granted the motion, it appears clear that the factual situation does not pertain to the instant case.