We have a moggie lad who has entered his amber yrs but has always had a sensitive tummy, after we learn't this we tried him on a light diet of Tuna chunks with no brine or oil, white fish and white chicken(no skin or fat) and he's been happy and healthy as have his poohs ever since
Sometimes though highly recommended dried ,specially formulated foods though of a high,quality standard can make things worse and as for feeding them together maybe try the above or if possible feed them seperatley in different rooms, as sometimes just coz your cats aren't physically killing each other on sight or being obvious in other ways, doesn't always mean they're best buds,they could be making each other anxious without your knowing and this can add to the problem. I know it's hard and frustrating as we have only one oldish man now but we used to have 2(1 plays at rainbow bridge) and though they lived,argued, played and ate together for years(to see them you'd have thought they idolised each other and sometimes they did) they both had tummy issues,when 1 passed our other chap improved no end of course combined with his light diet
Just try different diets,don't give up-you'll find a solution that suits all 3 of you i'm sure