Sounds like it is time for a trip to the vet for your big guy. It is possible he has a parasite or something else is going on...better to get it checked out, hope he is okay. If he is okay, you might just want to put her food up high, where he cant jump to...

One thing that works for some is to feed the smaller cat in a large, thick-walled cardboard box that has a small hole cut into the lower side just large enough for the smaller cat to get in and out. The larger cat will not be able to fit through the hole and that the smaller cat can nibble and come and go as she pleases, but keeping the big guy from gobbling up all her food. Another technique is to put a chain across a pantry door allowing just enough room for a smaller or thinner cat to squeeze through. Same idea. If the bigger cat is morbidly obese and can't jump, you can feed the smaller cat on the counter. A cat has to be pretty obese, though, to not be able to jump onto the counter.

Obesity is a very serious problem in cats, and many will go on to develop type II diabetes, very similar to what is happening to people all over the US. Obesity can lead to many other illnesses as well, including fatty liver disease, joint problems, dermatologic problems, heart disease and there is also an increased risk of cancers. It would be really good to get his weight back down.

Anytime a diet is changed in a cat, there can be diarrhea. I know nothing about this blue diet of yours, but Friskies is a decent diet, so if he does better on that, I would switch back.. and if you are struggling with his weight, cats loose weight a lot more easily on canned than on dry.

I would quit free feeding and if necessary separate them at feeding time. That way you have control over how much they eat.
As for the diarrhea - if it doesn't resolve itself in a couple days of eating normal amounts of food - I would take him to the vet.
Blue Buffalo is a good food. Much better than Friskies.
Katharina

10 years ago I free fed too. Some cats manage well that way, some don't. When one of my males was up to 17 pounds I switched to portion control and have never gone back. It took more than a year for him to get back to the correct weight, but no one suffered and all learned to eat their portions from seperate bowls twice a day. It may take a week for them to get used to the change, and they might be noisy about it, but it's healthier for them and they will adjust!

Friskies dry is mostly corn, (cats need meat) so good you've upgraded their diet.

as I said, canned food is the way to go.