It's hard to say what could be causing the problem - it could be the change of diet, although it sounds like you've done everything correctly there. It could be a blockage from a swallowed piece of toy, string, hairband or any number of other things you would never think twice about - until it ends up in your kitty's tummy.
There are so many possible reasons for a kitty to throw up, it is tough to know when to worry. I've had cats throw up for 2-3 days in a row then never again. Unfortunately, an initial vet screening is not always helpful - even an xray is not going to necessarily show a blockage if the object is not metal or calcium based, and for whatever reason vets are reluctant to jump right to a barium swallow, which is much more accurate in finding a blockage (but still not 100%).
I think what you need to evaluate is how much is kitty keeping down vs. throwing up? I know that's difficult with two of them but you probably have a fairly good idea of how much they eat as a pair - split that in half and compare to what has come up. Also, very important, make sure that kitty doesn't become dehydrated. Pinch the skin on the back of the neck - if it doesn't immediately spring back he may be dehydrated and need additional fluids. Sometimes simply rehydrating a cat will increase his appetite and get him to start eating again.
And finally, even if the vet says everything is okay, I would still keep a watchful eye on them, and if things don't improve it warrants further investigation.