Grit: provide from a very early age. It is small rocks to help grind the food in their gizzard.
Oyster shell: only needed by hens who are actually laying, because it's to provide calcium.
For both of them: do not mix into the food. Do not mix with each other. Put each in a dish or feeder of...
Feed, water, and grit are all they need.
Be sure they have access to plain water with NOTHING added.
If you want to add electrolytes or vinegar or something: that can go in a second waterer, but not in their only source of water.