Why torpedo-style contra-rotating propellers do not scale easily

Could contra-rotating torpedo-style propellers transmit more horsepower through two capital-ship shafts?

Not easily; ship-sized propellers move so much water that the resulting forces create severe mechanical and structural problems.

Contra-rotation is manageable at torpedo scale because the propellers move far less water and transmit much smaller loads. A capital ship's propellers must accelerate an enormous mass of water to absorb the machinery's power. Scaling the concept therefore sends much greater force back through shafts, bearings, and gearing. Those structural and mechanical penalties outweigh the apparent advantage of putting more horsepower through a two-shaft arrangement.