Presuming the IAMS kitten food is canned? (Hope so! If not, ditch the dry!) Compare the labels of the kitten food and adult food. You'll probably see the kitten food is higher in proteins and carbs. Also compare the phosphorus amounts. This link will be very helpful - it tells what the percentages should be for a healthy urinary tract.
Feline Urinary Tract Health: Cystitis, Urethral Obstruction, Urinary Tract Infection by Lisa A. Pierson, DVM :: cat urinary tract health.
Do you free feed? That is, do you leave the food down and let the cats pick or do you have set meal times and the cats generally finish their meals at one time? An option would be to pick up the dishes after a set meal so that the older cat wouldn't have access to the kitten food. Or is it possible to feed the kitten somewhere out of reach of the older one?
I feed set meal times, but I leave the dishes down so the cats can pick at whatever they don't finish. But I have one younger cat who can't eat canned food (we have not resolved why it gives her severe diarrhea, but working on that), so she has dry available. However, I have an elderly cat who cannot have even one kibble or his UT goes haywire. He won't jump up on a bathroom vanity, so that's where the dry is.