It's a typical natural battle for breeding supremacy, you won't be able to stop it so all the above advice is spot on. Seperate them and depending upon how old the elder cockeral is, as already said he might still be fertile or he might be best suited for the crock pot.
Without wanting to start a whole kill & eat or not debate, I'd kill the old boy and make use of him as a table bird. After all, is that not what we are all about as either smallholders, aspiring smallholders or self -sufficientists? Do it humanely and enjoy the bird a second time over as it were (& use the carcass for stock & freeze it for later use!). Job done, no waste.