I believe I understand what you're asking. You received chicks that had parents that were Cornish and that were Plymouth Rock. They were purchased as "broilers" and were for eating. You kept some of the chicks. Those chicks had babies. If I've said what happened correctly, the babies will be a mixture of Cornish type and Plymouth Rock type and everything in between.
You will get some that are very much like the Plymouth Rock, some that are very much like the Cornish, and some that are like the combination.
Unfortunately, you are getting two eggs a week. This probably indicates that they are not very fertile, but they were not meant to be since they were purchased as broilers. It could also mean that the ones you kept are more like the Cornish (which don't lay a lot of eggs).
It is a fun experiment, but I always try to weigh what I get in return for what I have. If my birds are for complete pleasure, then I don't care. I keep D'Uccles, try to hatch all their eggs, but don't get many (because I just don't do good with hatching). They are simply for pleasure, and I adore them.
I also keep Ameraucana, Orpington and Plymouth Rocks. They are for pleasure, but they are also for eggs. Those birds that are not laying often are not worth as much to me because I keep them for egg production. At the cost of feed, nearly all my birds must give me, at least, what they cost me back because I can't afford to pay for feed for a lot of birds that don't return something. Although the Orpingtons make a decent broiler, I don't think that the Ameraucanas do. When I took my Plymouth Rock roosters that I could not keep to a chicken processing facility, he told me not to waste my time growing these birds because they were scrawny. I was shocked at his comment, but he processes birds for meat, and that was his view.
If you kept the birds to see what you would get, then that will be interesting. You actually might get close to duplicates to each parent i.e. some that are nearly pure Cornish and some that are nearly pure Rock. If you keep the father that is most like a Cornish, and the mother that is most like a Rock, you might get to hatch chicks and then keep the babies for broilers. But then again, I go back to figuring what the cost for me is. And after I pay for broilers, it is cheaper to buy the chicks than to keep parents to try to raise my own at the cost of feed.
It would be interesting to me to see what you got if you kept a rooster most like a Cornish and a hen more like the Rock and bred them. I wonder how close you could get to the parent stock.
