A JUDGE said he would need a crystal ball to know for sure whether two female illegal immigrants would honour their undertaking to return from China to testify in a criminal trial. Judge Caird made the comments in the District Court yesterday, in the case of Cheng Fai-tang, 18, and Poon Shek-kin, 16. They face two joint charges of exercising control over a woman with a view to her prostitution, and two joint counts of aiding and abetting an illegal immigrant remain in the territory. Cheng is also accused of living off the earnings of prostitution. The detention of the two key prosecution witnesses - Au Yeung Ling-ling, 14, and Tong Hiu-kwan, 17 - sparked an outcry when it was revealed they had been detained in Hong Kong since their arrest last August. In May Governor Chris Patten ordered that the girls be removed from Victoria Prison into the care of the Social Welfare Department. On June 29, Jim Chandler, then Cheng's defence counsel, indicated he would apply for a stay of proceedings on the grounds of an abuse of court process. Three days were set, starting from yesterday, for arguments on the stay. But Cheng's new counsel, Kenneth G. M. Chan, yesterday said it was decided on Friday that he would not proceed with the application for a stay of proceedings. Prosecutor John Reading said the two illegal immigrant witnesses had been repatriated to China on July 11. He said although the two girls had undertaken to come back to Hong Kong to testify in the trial of Cheng and Poon, four months was needed to enable their return from Hunan province. Judge Caird extended the defendants' bail until their trial on February 13. He noted that the Crown had taken reasonable steps to ensure the girls were not kept unduly in Hong Kong. But he said it would have been better, with the benefit of hindsight, if the matter had gone ahead on the original trial date of July 7.