Abbie is very jealous of Sammy and growls and snaps at him How can I change this behavior

has anything else changed in the house. Did you go on vacation? is someone else coming to the house more often and showing more attetion to one and not the other. there is a number of things that can cause this you need to look at every little thing. it could be as simple as your not home as much and they are acting out for attention

You really can't begin to deal with an issue like this until you find the underlying cause. There are many reasons for aggression, and different types will manifest in different situations.

Is she aggressive with anyone or anything else? If so, this would lead me first to the vet, to see if she is in some kind of pain (sick or injured).

Next, I would look for when the aggression is occurring. Does it happen only over food, toys, or sleeping areas? Does it occur during all of those? Some other time? What are the circumstances surrounding the incidents?

I can't in good conscience give advice on how to deal with a situation like this without knowing all of the details. There are too many possiblities, and proper correction will depend on those details.

THE CONVENTIONAL ADVICE to figure out which dog is or should be the higher ranking one and to treat that dog preferentially like having more privileges and petting time.

Normal fights occur over normal causes, mostly some valued resource (a toy or food or being center of attention from owner), something both dogs want but is in limited supply (or dogs think so) so that they can't both have it.

For dogs in same household where both are fairly normal (confident) dogs, the fights are almost always about social status like dominance, the right to be preferred and have first choice over food and toys. Understand that dogs do not believe in equality.

For such situations, the conventional advice to give preferential treatment and support to the higher ranked dog. You should allow dogs to posture at each other unless the situation seems to be escalating. If it is escalating, that is time to interrupt the situation. (Which I would follow by a long down-stay for both or crating for both.) Don't allow or encourage the subordinate dog to challenge or aggravate the dominant dog.

Also, you may inadvertently be causing them to fight. You need to assert yourself as the alpha and treat each dog accordingly not tolerating any infractions. I do this with multiple fosters and separate fighting with a large plastic Rubbermade lid. When dogs are "posturing" during play, a fight may ensue shortly afterward so keep a tennis racket or anything you can use to separate them.

You may need a professional trainer for this, but I also flip and pin my dogs (a sharp "No") that show aggression holding them down until they totally submit; I also push the other dog away during the correction. I continue this every time the aggressive dog does this, but after three corrections the dog gets a timeout away from the other dog. Alone time is a good thing.

When he submits, I follow it with praise. I never hit my dogs or subject them to any pain, but I do let them know that infractions within the pack will be dealt with quickly.

You Cocker Spaniel needs to get out more, exercise, and socialize with other dogs to learn how to interact within the pack. I find dog parks and doggie play dates so valuable.

Desensitize your dogs with plenty of toys, chews, and play dates along with a strong sense of dog savvy ownership, and you should see the aggressive behavior diminish.

If it continues, a hands on professional trainer can deal with this in a few sessions.