I agree with Eldartford with the caveat that if you run both conductors from the amp to the given speaker you will be doubling the cross section of the , thus providing a slightly lower resistance and reducing I^2R losses. This will likely not have a noticeable effect as your speaker wire is probably no smaller than 16 guage and you likely are not passing enough current through the conductors to make a difference.
As to why some companies use multiple conductors in a cable that is meant to be connected at only one point on each end- the proffered theory is to reduce the impact of skin effect - which is the tendency for current density to be greater away from the center of a conductor. Technically true - but skin effect is frequency dependent and the flaw in the proffered theory is that at audio frequencies the impact of skin effect is negligible.
As to why some companies use multiple conductors in a cable that is meant to be connected at only one point on each end- the proffered theory is to reduce the impact of skin effect - which is the tendency for current density to be greater away from the center of a conductor. Technically true - but skin effect is frequency dependent and the flaw in the proffered theory is that at audio frequencies the impact of skin effect is negligible.