Good comments by the others. I would add that the only situation in which I would definitely avoid doing that is if the cable happens to be one of the few cable types that have ultra-high capacitance, such as Goertz, or the old Polk Cobra cable. Paralleling two identical runs, as you are proposing, will double the cable capacitance seen by the amplifier, and if the capacitance of the cable is very high to begin with that could result in adverse effects up to and including oscillations and amplifier damage.
With the great majority of speaker cables, however, the doubled capacitance will not be of any significance, and the halving of resistance and inductance that will result will (at least slightly) work in the direction of improving the accuracy of transmission of the signal, as well as resulting in a slight increase in woofer damping. Of course, those differences may not always be subjectively preferable, so as suggested above it may be a good idea to try it both ways.
Regards,
-- Al