From what I have read cold flashing is not easy, timing of exec the command and connecting the device is critical.

I m just throwing out a suggestion here ...the bug link that I posted around the 20th comment a kernel with the framebuffer enabled is posted. That should give you a verbose output rather than the flashing dots and you will be able see where exactly the system is hanging.

I would flash the combined then the emmc then the combined and then the other kernel. But again im talking from no experiece with this sort of thing just from what I understand. Hopefully someone with experience could clarify.