If you get them from a hatchery or feed store, the Cornish rocks and Cornish X are the same "breed" just under different names--they are a fast-growing hybrid cross of specialized breeds (aka they are not really a Cornish crossed with a White Rock) used for meat production. Unless they are kept on a very strict diet, they often develop internal health problems and leg problems, especially as they age. Many do not live past 1 year. If you can keep their diets very lean and encourage free ranging and exercise, some Cornish X can live longer, even old enough to produce eggs. They are not very good layers and, by egg-laying age, their feed to egg laying ratio is not good either. The roosters often have trouble mounting the hens, so fertility would not be high and a lighter cock would be needed for breeding. However, I do know of a few people that have had success with using the Cornish X as a dual-purpose breed, just don't expect to get an egg every day or every other day.
Btw, we usually slaughter our penned Cornish X at 8 weeks old since they tend to have more trouble in the heat after this age. If they are free range, you can wait a little longer.