. . for bridging, routing and other functions is actually done by MAC address, having several cards with the same address would probably make quite a mess of things.
if they're not online at the same time (as he was talking about).
He said that when he unplugged one from the CM, and plugged another in, that the CM didn't work. Which would indicate they're not online at the same time, and therefore irrelevant to the MAC address.
If they're on a network together, whole new ball game.