I have a MacBookPro 15" with an integrated gpu GTX 750m and an eGPU GTX 950.
I tested the 2.78 builds from build-bot and seems that the gpu it is recognised only as a OpenCL device and crashes on Cycles render initialisation. No CUDA option to select.
Blender 2.77a recognize and use properly the CUDA with the both Nvidia cards..
