Never said it it was impossible. All I said was it's less efficient. And it is. No matter where you mount that pusher, it is disrupting the incoming air. Maybe not enough that it's causing you to overheat, but you can't argue with the laws of physics. Your mechanical fan and radiator would work better without the electric fan disrupting the incoming air. Your electric fan would work better without the mechanical fan disrupting the air it's pushing through the radiator. If running both is working it just means you have enough cooling capacity when the car is moving at speed that you can sacrifice some efficiency without overheating, because that is what you're doing. And that makes sense, since you don't have a fan clutch on your mechanical fan you're spinning it all the time, so you probably do have some extra cooling capability at speed. Of course, you also have unnecessary parasitic loss, because you're spinning a mechanical fan that you don't need in that situation.
Not to mention the added complexity and weight of running two fan systems, instead of just running one that has the right capacity and capabilities for your car. If you're going to add an electric fan, you might as well size it so you don't need the mechanical fan. Same for the mechanical fan, if you're going to run one you should make sure it has the capacity to cool the engine. And that means the right shroud, fan spaced correctly, fan clutch so you're not wasting horsepower when you don't need to, and a fan that pulls enough air. Otherwise you're just running two undersized fan systems, sacrificing efficiency and bumping up your parasitic losses.
