Hiya
Not sure what the difference is with the foods but I adopted a cat a few months ago and when he got to 11 months I started him on the adult.
For those few weeks I bought him the kitten food he eat his and then he went to the other bowls and eat theirs and never had an tummy upsets.
Has us said they eat for England.
Mine still does and here's one now!
What you could do is give them adult food for one meal and then kitten food for their second meal of the day.
When i feed the street cats in Turkey the kittens join in and they all seem to thrive on adult food.
Hope this helps.