Your magnetos will keep your plugs sparking, but you'll lose control of your props. They will freeze at whatever pitch they're in at the moment, and you'll be flying a fixed pitch airplane, so be ready to switch to a different set of throttle reflexes. (Unless, of course, you are lucky enough to be flying one of the models equipped with hydromatic props!)
Your propulsive efficiency will suffer badly, and you won't be able to achieve full power without overspeeding your engines, or, depending on what airspeed and power setting you were at when your props froze, you may not be able to achieve redline RPM at all, seriously limiting your available horsepower and probably overtorquing your engines.
If you're engaged in combat and have dropped your tanks, you better disengage immediately, head for home, and check your survival equipment, because you'll probably be swimming. Your MPG with fixed pitch props will suffer badly. Add to that, no radios, no electronic navigation (RDF), no armament firing or dropping circuits, and you're in for more fun than a barrel of monkees!
WHAT IDIOT specified that setup?? I did several training flights in a 150 HP Apache with one generator on the right engine and one hydraulic pump on the left. Either engine out in IMC with a sharp, proficient pilot was a barely survivable situation; with your average complacent pilot, a recipe for disaster. I never ever worked up a sweat in the air like I did in that Pigache.