Many current models of double 18 inch Subs have a total power capacity that exceeds the current capability of a simple pair of even AWG #14 guage.
Remember the ratings of our wire are based on electrical safety standards. A AWG#14 guage wire can handle 20 Amperes of current without heating up to the point that the insulation will break down and cause a short or worse . . . a fire.
This does not mean that at a given current level the wire will NOT be Hot. If you loudspeaker cable is hot, you have significantly raised the impedance of the wire. This impedance is seen by the loudspeaker as being in series with the internal source impedance of the amplifier and thus adversely affects the DF.
A dual 18 Sub that handles 2400 Watts continuously would be drawing 24.5 Amperes of current at that continuous power. This is too much current for any significant length of AWG #14. Using two independent runs to each loudspeaker will split the current to 12.25 Amps per run. The wire stays cooler and what ever the DF may be it is constant and hopefully higher than 20 as a system.