A saddle stitch is sewn with one thread and a needle at each end. Both needles go through every hole, one from each side, so the thread crosses itself inside the leather and every stitch is held twice. Cut one and the seam hardly notices: the other half of the thread is still sitting in each hole, holding on. It takes longer than going through once, and the extra time is not what the strength costs. It is the strength, done twice on purpose.