Assuming that the woofers are identical, they will have the same frequency response. However, when using two subs in the same room, it is possible to generate somewhat smoother low frequencies due to the effect of the room acoustics. A single subwoofer may excite resonant modes in the room which interfere with the acoustic output of the sub, thereby producing phase cancellation, or nulls, at certain frequencies. A pair of subs, however, will often acoustically load the room differently than a solo sub, so the effects of phase cancellation are reduced, thereby increasing the output.
There is another thread below, "Subwoofers and standing waves", that explores this topic as well.
