Ok, one half gets F=pπr2 because the area is effectively just the one of the equator (right?). The force that holds together the two halves is the circle 2πr times the tension T. So you get p=2T/r. Then you have exactly the same calculation, but with a 2 instead of a 3: V = (Nkτ/2T)3/2(4π/3)-1/2...