Great question! It's recommended that the grit and the oyster shell be offered separately. We don’t know exactly how much a hen will need in any one day, but she is pretty good about regulating her own intake on those things. If the grit and oyster shell are mixed in with the feed, she may scatter the feed all over either to get to a particular piece of grit or shell or to avoid a particular piece of shell.
They really do have preferences on which bit looks good! So if it is all mixed in, and she doesn’t need it, she will kick it aside and probably waste some feed.
Having it out in a separate container or area works well because it is up to the hen to choose if she wants it or not. Different pieces dissolve slower or faster inside the bird, so if she eats a big piece of shell one day she may not want any the next day.