[Krupp]

from the "Hitler's U-Boat war, The hunters, 1939-1942" - by Clay Blair:

"Stern compartment was known as the electrical room. It contained two 375-horsepower electric motors, or more precisely, motor-generators. The main driveshafts of the diesel engines ran through the core of the motor-generators. When the diesels were operating, either or both motor-generators could be clutched onto the turning driveshafts to serve as generators to charge the batteries. Or, as a fuel-saving (and range-extending) measure, the power produced by one motor-generator, operated by one diesel engine, could be routed to the other motor-generator to turn the other shaft (so rigged, the maximum cruising speed was a leisury 6 knots). Upon diving, when both diesels were shut down, the motor-generators were clutched to the driveshafts, drawing power from the batteries."

So the german U-Boats could drive two prop-shafts with one diesel engine when other diesel powered the generator to charge batteries.