Would likely reduce the performance. The powerline units would still send the signals directly between each other. You have just now added another device to compete for the bandwidth. They tend to operate more like wireless than say a ethernet cable and switch. You could I suppose use 2 pair on different encryption keys but they still share the same total bandwidth in the house wires. You would have to do something silly like plug 2 units into the same outlet using different keys and then connect the ethernet ports together. I suspect you would have massive issues.