This possibly happened to all cat owners: "If I put kitten food down and regular food for the older cat, is there a good possibility they will eat each other’s food? And if so, is there any harm to that?"
If you serve up both kitten and adult food, chances are the adult will eat most of the kitten food along with the adult food, which could result in a cat with a weight problem and a kitten not getting all the advantages it needs.
Kitten food tends to be higher in calories, but it also contains some vital nutrients that a growing kitten needs. It won’t harm the adult cat to eat kitten food, other than consuming the extra calories, but the kitten definitely shouldn’t be eating food formulated for adult cats.
It would be best to feed them in separate rooms and pick up the food after they’ve eaten. That might be difficult because cats, unlike dogs that quickly wolf down their food and lick their bowls clean, tend to be nibblers, eating a little now and a little later.
There are cat foods that are good for all ages, so talk to your vet about which one might be best for your cats. The good news is that kittens only need the special diet for about a year, unless it’s a larger breed cat, such as a Maine coon, which takes about 18 months to reach maturity.
Depending on the age of your adult cat, you might need to start considering age in choosing an adult food. Older cats tend to develop teeth and gum issues as they age and might require a softer kibble or canned food.
Vets used to recommend lowering the amount of protein senior cats receive, but studies now suggest it’s the phosphorus in their diets, not the protein, that can lead to kidney disease. Older cats probably don’t digest protein and fats as well as younger cats do, so they might need those nutrients in a different form.