The definition of detection on defectives is given，which is suitable for both distinctive defectives and nondistinctive defectives. This paper is focalized on the scale model with two defectives under a bi-test de-vice. A search schedule is presented and proved that it is optimal by measure. At the end of this paper it is pointed out that the search schedule mentioned above is not uniformly optimal and a guess is made that an informational optimal search schedule maybe exists.