Don't Make Me Release The Flying Monkeys

One of my two cats has gotten too tubby. He was always pudgy, but you could officially classify him as fat now. Meanwhile, my other cat remains trim and muscular. They're both middle-aged, but metabolic changes aren't effecting them at the same rate. They always eat together off their shared plate and there's no easy way to feed them separately without epic drama, I'm certain.

So my question is: can I safely feed both cats reduced calorie food without the normal-weight cat getting underweight?

Meow (means thank you)