Basic physics: Transfering energy results in the reduction of any one givin form of energy.
As the energy transfers through the middle uni, it if fact transfers multiple times through each pivotal points, through the bearing, through the oil and so on. While doing this the kenetic/torque energy has to change directions constantly, thus losing momentum and also transforming some of the kenetic/torque energy into other forms of energy such as heat, sound and light.
Single shaft = a more direct route for the energy to transfer through, and gives a greater opertunity for the energy to reatain its momentum.
The mutiple shafts do reduce virbration as shorter shaft give less oppotunity for imbalance and also reduce potential imbalance multplier which increase exponentially with the legth of the shaft.
Single shaft = more derict raw energy tansfer.... more power :y:
Multiple shaft = more refined energy transfer between the axle and engine.... more comfort.