Having multiple pets can be a very rewarding experience. However, it can definitely have its challenges. Finding time to provide each pet with an appropriate amount of play time and exercise as well as handling intra-household behavioral issues are just some of the concerns encountered. One additional issue is making sure each pet is eating their appropriate diet. Dogs may get into the cat food, the diabetic cat may get into the regular cat food, and so on.
One major way to solve this situation is feeding every pet separately. Most of the time, there is an issue because one or more of the pets have food left out throughout the day, whether it is intentional or not. The best way to feed each pet separately is to have timed meals, where each pet is offered their food bowl two to three times daily (three times daily allows for smaller meals) for a set amount of time, usually 10 to 20 minutes, and then that meal is taken away. If the pet is used to having food out all the time, this may take some getting used to. Eventually they will learn to eat their entire meal in the appropriate time. Not only will this allow you to feed each pet their appropriate diet, but it also allows you to control how much they get, and hopefully prevent them from putting on excess pounds. Sometimes it is necessary to separate the pets from each other. This can be done by separating them into different rooms, or even feeding dogs in a crate. This article at Veterinary Partner has more detailed information on how to do this:
If the only concern is trying to prevent the dogs from getting into cat food, the solution may be more along the lines of where you place the food. Cats are typically smaller and able to get to more places than dogs. You may be able to try putting the cat food up on a counter, or even in a room/ area that is sectioned off by silhouette boards that only the cats can get into. This works best if the cats are doing well with free-feeding, but the pesky dog keeps helping himself on their tasty food left out. If it is the cat trying to eat the dog food, it gets bit trickier trying to find a place that the cat cannot get to but the dog can. I would recommend in this case trying out the separate timed meals described earlier.