Well, that helps. Since you said (presumably after testing them individually) that one motor and one ESC is now dead/not working, it sounds like the motor killed the ESC. That can happen if the motor was already damaged. Perhaps by a screw used to mount the motor or some of the loose metal that might have come from the aluminum can type motor mount got into the motor and shorted out some windings. That will kill most ESC's eventually (specifically the FET's). I'm guessing it most likely would have been the motor/ESC pair you hadn't tested prior to connecting them together.
In the future, if you can, I'd recommend testing individual components separately if possible before using them together. I do that with all my servos, esc's, motors, etc. I mean, I will pair an esc/motor, but if I were making a twin, I would test each individually before I connect them together. Just much easier to troubleshoot in the future.
Goes with the KISS principle which means "Keep It Simple and Straightforward" (that last S also can be "stupid") and follows what I suggested above about keeping your wiring as simple as possible.