The federal judge overseeing the trial of Mayor Marion Barry on drug and perjury charges is expected to rule tomorrow on the pivotal issue of whether to split the case into two trials, a move he is reluctant to make, according to sources familiar with the case.

U.S. District Judge Thomas Penfield Jackson has scheduled a 3 p.m. hearing tomorrow on the issue. He has given defense lawyers until that time to decide whether they prefer to defend against the full 14 charges contained in the second, larger indictment returned May 10 or to postpone a trial on the six cocaine charges, including one of conspiracy to possess cocaine, added in that indictment.

Altogether Barry faces three counts of perjury, 10 counts of possession of cocaine and one count of conspiring to possess cocaine. Jury selection in the case is scheduled to begin a week from today, June 4.

If Jackson agrees to separate the case, it would give defense attorneys R. Kenneth Mundy and Robert W. Mance the strategic advantage of defending the mayor against the perjury charges, which are felonies, without having to confront directly the conspiracy charge, which is a misdemeanor.

The Washington Post reported yesterday that U.S. Attorney Jay B. Stephens has provided a list of 19 unindicted co-conspirators to Barry and his attorneys that includes the names of longtime friends, business figures and former girlfriends.

Barry was originally indicted in February on the perjury charges and five counts of cocaine possession arising from the Ramada Inn episode of December 1988 and the mayor's arrest on Jan. 18 this year at the Vista Hotel. Legal experts consider Barry's main defense in that case to rest on a comparison of the mayor's credibility with that of the chief prosecution witnesses, former D.C. government employee Charles Lewis and FBI informant Hazel Diane "Rasheeda" Moore.

Sources said defense lawyers are concerned that if they are faced with a conspiracy charge with 19 or more witnesses, it would be difficult to mount the same type of attack on each person's credibility.

If the defense lawyers decide to press for a separation, or severance, of the charges, Stephens already has filed a written response that vigorously argues that the case should be kept together and that Mundy could be granted a brief postponement -- of one to two weeks -- if he needs more time to prepare his defense.

Meanwhile yesterday, the attorney for one woman on the list of alleged co-conspirators, Doris Crenshaw, confirmed that she was cooperating with federal prosecutors. The Washington Post incorrectly reported yesterday that Crenshaw was a former girlfriend of the mayor's. Her attorney, Jay Cooper, said Crenshaw "has known Barry for a long time, dating back to the civil rights movement days, and has been a political associate."

Crenshaw, who owns a public finance consulting business in Montgomery, Ala., declined to be interviewed, but Cooper said she has been questioned about a visit Barry made to Crenshaw's room at the Mayflower Hotel during a trip she made to Washington in November 1989. Sources have said Crenshaw told investigators that she saw Barry use cocaine during that trip.

In an interview with The Post while she was in town in November, Crenshaw said she knew of no drug use by Barry.

Also, sources said yesterday that another alleged co-conspirator, advertising executive Jeffrey Mitchell, owned a half interest in a firm that received a $24,987 contract from the D.C. Department of Human Services in 1987. In an interview Saturday, Mitchell said he was not surprised to be named as a co-conspirator, but he said he had never seen Barry use drugs and had never obtained business from the city.