Nick, joining two 16ga wires would give you the equivalent of a 13ga wire. There's at least a theoretical disadvantage in using two completely separate speaker wires; unless they were twisted tightly or otherwise bound closely together the distance between the conductors would be wider than normal, which would increase the inductance and cause a slight rolloff of the high frequencies. This might not be audible, but why bother? The 16ga is plenty.

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Enjoy the music, not the equipment.