No, power = V^2/R. R halves but V stays the same so power doubles.
Or looking at current, Power = I^2 * R, current increases by 2 but resistance halves so 2^2 x 0.5 = 2, power doubles.
The other half of the volume increase comes because you've doubled the area of the speakers.
This is why a 4x10 can be louder than a 1x15, all other things being equal (they never are!) and a 2x12 is twice as loud as a 1x12 for the same power (assuming again that everything else stays the same).