Have you talked with your vet about the raw chicken food many pet stores offer? I know quite a few people who swear by the raw chicken for cats. My MIL prepares her own, however, I understand that Petco will soon be carrying them.
Be very careful about your big cat "living off his fat". This can cause feline fatty liver disease and affect both the liver and pancreas. When we first got our cat (he was 8 years old then, 16 now) he had stopped eating because of stress at his old home. This is considered Feline Anorexia, and because of this he developed feline fatty liver disease. We really weren't sure if he was going to make it for a few days, he was in BAD shape. We ended up tube feeding him for six weeks+ follow-up care, to the tune of about $4, 000 of vet bills when all was said and done. As a very senior cat now, he has chronic kidney failure and pancreatitis (inflammation of the pancreas) that we deal with. Because of the kidney failure, he goes in to the vet for a bag of IV fluids and vitamin B-12 acupuncture every so often (we go to a holistic vet; the conventional vet wasn't quite working for us and only made him sicker... he's gotten a lot better since.)
He now gets Wellness wet food (chicken and herring, which he loves...and lots of water mixed in) and a regimen of granules for indigestion or Pepcid AC for acid (which makes him nauseous-- which is why he wasn't eating), a tincture with slippery elm and some other herbs and a laxative, because he's pretty dependent upon it at this point. All of this is mixed into his food. The old vet had given him an appetite stimulant which he had a strong seratonin syndrome reaction to; for us, the Pepcid actually works better and didn't tweak him out the way the medical drug (an anti-depressant) did. Many pets respond well to that medication, our old Gus did not.
I agree with others that getting blood panels done would be my first step in your situation, and then go from there. We had Gus before we had our son, so he was our 'baby' for a while. Now we are mostly just wanting to keep him comfortable as he ages. 16 is a long, good life for a kitty. Sorry this is so long, and I hope your cats feel better soon. (Maybe feeding them in separate rooms might work, if they need different food?)