Sounds like a toxin of some kind, they're aren't a lot of illnesses that will cross between dogs and cats. Have they gotten into anything in the house that they shouldn't have... Foods, medications, vitamins, cleaners? Have they spent time in the garage where they could have lapped up something that they shouldn't have? Have you gotten any new house plants that they both might have chewed on? Has your house or yard been treated/sprayed for insects recently? Did you recently put out snail pellets... Or fertilizer granules? Whether or not you can figure it out the cause, you need to get them to a vet. If it is a toxin of some sort, the longer they go without treatment, the greater the chances of irreparable organ damage being done.