Muilt Diplay and CUDA problems bug or something else?

when I using two dispaly, the performance of CUDA is about half of the origin (single display);
i think it must be that the cuda sparate the hardware resources for two displays;
so I config the two App by the follow mothods, but the seconds app is exit in cudasetdevice()function. why ?anyone can tell me the reason?
first App _________________________second app

glutInitWindowPosition(0, 0)glutInitWindowPositon(1024, 0)
-------cuda initialize _______________ ------cuda initialize
… ___________________________ …
…cudaSetDevice();–error and exit???
…same part
_________…same part