It is actually better to wire the voice coils on each sub in series and then wire the 2 subs in parallel to the amp. . .
A little experiment: Wire 2 subs in series. With them sitting on the floor in free air, move the cone on one sub. . .the cone on the other sub will move by the current induced as the first sub moves. Won't happen in parallel. . . .
Another reason would be you now have a pair of speaker wires run to the drivers for less power loss. . . .although this power loss should not be noticable with decent wire. . .