Actually, if you go back and look closely at the TSC link posted previously, you can see that there are two sets of holes, evidently for some flexibility in lining up the holes.
Each set is through the center of the adapter - and consequently the center of the pto shaft. At least half of the pto shafts that I looked at on the used tractor lot had holes in the pto shaft, just for this purpose I presume.
I asked the TSC expert why there wasn't an extension with a spring-loaded pin like on the driveshaft, and he opined that it might not be a good idea to have two of them together like that. /forums/images/graemlins/confused.gif