You should never separate mating dogs that have tied, it could cause injury to both of them. If you do not want the two dogs to mate, you have enough time to separate them before. When they are breeding, the male's penis swells inside the female's vagina, and thus, it can make them stay stuck to each other after mating for a while.  It can take around 15 minutes to 30 minutes to remove themselves from each other. It is advised to never interfere when the two dogs are already mating.  Do not try to separate them since it can lead to physical damage to their genitals, and you might have to face intense aggression from the dogs.