The ingredients that make up the flea collars are different for both cats and dogs and if you use a cat flea collar on a dog then it could kill your dog and if you use a dog flea collar on the cat it could kill the cat.
Always use the proper flea collar for your dog and for your cat to avoid any health issues and potential death in your cat or dog.
There's a very good reason why they sell different flea collars for cats and dogs because cats and dogs are made different and require different medicine as well as flea protection.