Assuming there would be no problem physically accommodating the cable terminations, the only situation I can envision in which that may not be a good idea would be if the cable is one of the few types that have ultra-high capacitance, such as Goertz. Having two runs in parallel will double the total capacitance, which may be a problem for the amp if the capacitance of a single run is already very high. Two runs in parallel will also result in resistance and inductance being cut in half, which can only help assuming the goal is to minimize the sonic effects of the cable.
I had an expensive pair of bi-wired cables and then bought speakers with single wire. A phone call to MIT assured me that I could connect both pairs of the bi-wired wires to the single wired terminals on my speakers. I used it this way for over 5 years with no suffering of the sound.
+1 I've seen and heard this done many, many times with no ill effect.
Clear Day Cables offers speaker cable in a few different models, the only difference between them being the amount of wire in the cable. There is the Standard, then the Shotgun version (which doubles the wire), and the Double Shotgun (which, duh, doubles it again). Jeff advises that the longer the run and/or the lower the impedance of the loudspeaker, the heavier the gauge of the wire.
Different Clear Day owners/users prefer different versions, so you may as well try the biwire cables (if you get them) using just one leg, then both. You can decide for yourself which you prefer with your particular speakers and amp.