This is a common and easily rectified problem if you are willing to perhaps make some modifications to your cat's diets.

This problem can occur when a cat will eat too quickly (sometimes feeding from a smaller dish, so they take smaller bites and injest less air can help -very often these cats will eat their vomit, its normal and ok), or the diet simply does not agree.

I'm really not a fan of W/D, its a bulky, high fibre, totally unexciting, unappetising diet that usually causes the cats to clear the room after having a bowel movement. It is often prescribe for diabetics. The fat and protien content is also ridiculasly low, causing the cats to eat more in an effort to maintain their strength, I'm surprised that W/D was suggested rather than c/d -but again, I totally am not a fan of Hills in general (they still use ethoxyquin!).

Ideally, these cats would be much healthier and happier on a natural raw diet (incedently these diets help cats maintain low pH as its supposed to be) and certainly my male cat with chronic cystitus does really well on it (caution though, this problem may never totally go away until the root problem is discovered and cured which sometimes takes years to discover).

If you are really set against a raw food diet, at least switch to Wathams pH control which I'm sure will maintain your cats much better than Hills. At least Walthams doesn't try to starve these animals, their food actually offers substantial amounts of protien and better quality fats, and they are not nearly as bland and unappealing as Hill's diets. Keep in mind though, that many of these problems (especially urinary) are due to feeding grains to a carnivor. This is the truely unhealthy and dangerous practice here.