When he "divided his rope in half," it does not
cut it into two parts, each half the original length of the
doubt he simply untwisted the strands, and so divided it into two
each of the original length, but one-half the thickness.
He would thus
able to tie the two together and make a rope nearly twice the original
length, with which it is quite conceivable that he made good his escape
from the dungeon.