that is a good question. my thoughts on the two piece are out the window after this, one piece has two u joints to fix if failure occurs, two piece has carrier and 3 u joints. the carrier alone was almost 3 times what i paid for the shaft to be made (285 for the carrier 99 for the whole shaft)
I think they did the two piece because they had the parts, most toy pickups have a two piece that i believe is close to the same size in some models? the mkIV might be a one piece but dont know for sure.
The one piece def puts the power to the ground. if you decide to get one price one out at a driveline shop first. Mine is 2 and 5/8 diameter with .095 sidewall and they reused my front yoke and rear flange, even the old u joints because they were still good.
Highly recommend it