I had the same question in the beginning. Seems like you are giving them two kinds of rocks. But, they are different. The grit is not soluble. They need it for digestion. The oyster shell is for calcium for strong shells on their eggs. They do not function the same in a chicken.
BTW, I wouldn't add the oyster shell to their food. I would either scatter it around on the ground with the grit, or keep them in a separate dish.
Oyster shell should be left on the side in a separate container so the chickens can eat it when they need it. I would not mix it into their food. As for the grit, it depends on your soil. They need access to small pieces of rock, and young chicks specifically need the smaller crushed up kind. I would leave out a container of both at all times.
In the USA, grit is considered to be small stones, varying in size for youngsters and mature birds. oyster shell is necessary for laying hens, and should be offered in a separate dish, even if the hens are fed a layer feed.
In the UK, oyster shell is 'soluble grit', and small stones are called 'insoluble grit'. Semantics!
Birds do need insoluble grit, and some folks have enough available in the soil around the coop, and others don't. As soon as chicks are offered anything except chick starter to eat, they need grit, and it needs to be smaller than what's available to the adults.
I thought you had to have oyster shell available for digestive reasons but now I've had my flock for a year I guess it's more about the eggs quality altogether. So by the time my girls started laying I had come across a thread about using the egg shells instead of or with oyster shells. It's good for the girls and if you mix them they won't get spoiled if you happen run out of one.
My birds all free range but they do have layer pellets available in their coop and their eggs are always really sturdy and strong so I forgot about the egg shells/oyster I had prepared for the coop. I put it in a separate dish cause I've read they will only eat it if their body tells them they need it. They love it.
I don't always have it in the coop cause my girls have access to 2 acres of land and I've never had any issues with health or egg quality. If they are in a run it might be more necessary.
I don't feed a layers feed so I need to offer Oyster Shells for the calcium. I feed a Non-Medicated Starter-Grower 18% or All-Flock crumble 20%.
My chickens are kept in a pen 23 hours daily so most pebbles have been consumed.
My ground is frozen or covered with snow for a few weeks during winter season so I offer Granite Grit for digestion.
I keep a container of both available year round.
I average 7 pullets/hens, currently have 5 pullets and 3 hens.
I've had chickens nearly 5 years and I'm on my second bag of Oyster Shells 50 lbs and Granite Grit 25 lbs. Both are more than half full. Average around $10 bucks each.