A Montgomery County Circuit Court judge yesterday postponed for two weeks the murder and robbery trial of two Silver Spring men -- accused of literally frightening to death a motel clerk during a robbery -- after a third defendant agreed to testify against the two.

James F. Boyd, 20, of the District of Columbia, pleaded guilty before Judge Stanley B. Frosh Monday to felony murder and robbery and he agreed to testify in exchange for a life sentence with all but five years suspended. Under the plea agreement, additional charges of conspiracy to commit robbery and transportation of a handgun were dropped.

Melvin J. Alston, 26, and Ray A. Holman, 33, now are scheduled to go on trial Dec. 10 before Judge Irma S. Raker on all four charges, to which they both have pleaded innocent. Raker allowed the delay of their trial, which had been scheduled to start yesterday, after their defense attorneys said that Boyd's willingness to testify for the prosecution changed the complexion of the case.

The case arises from the death of Pearl A. Pizzamiglio, 60, a clerk at the Chevy Chase In Town Motor Hotel. Pizzamiglio, who was not physically attacked, died of a heart attack two hours after four men allegedly stole $167 from her on Nov. 19, 1983.

Last month, a Montgomery County jury found a fourth man, Michael Stewart, 20, of the District, guilty of robbery and felony murder for the robbery and Pizzamiglio's death.

Under Maryland law, felony murder is committed when a death results from the commission of another felony such as rape, arson or robbery. Aside from Stewart's conviction, felony murder convictions have been won in similar cases involving stress-related heart attacks in Arizona, California and New Mexico, according to a spokesman for the state's attorney's office.