No you shouldn't use a flea collar for a cat on a dog or vise versa since the formula that is in cat and dog flea collars differ from one another and if the wrong collar is used on the dog or cat then the wrong flea collar could prove fatal to the dog or cat.
There's different flea collars for dogs and different flea collars for cats for a good reason since cats bodies are different from dogs.
Always use a dog flea collar on a dog and a cat flea collar on a cat.