If you put it that way, because that is what I was suggesting, feed the Cons to the P's when the Cons are smaller, so the P's still get food, but its over quickly. What else do you feed the P's other than Cons?
Feeding live feeders is one thing, but tossing in a larger fish to see it tortured and ripped apart is why Piranha keepers have a bad reputation. If you no longer want your Convicts, freeze them and then feed them to your Piranhas. They won't be so shy against a dead fish.