Cocker Stalking and Fighting with other dog

Question
QUESTION: We have a mixed small hound and a cocker spaniel, both females and both spayed.  We had the hound for about 6 years, then got the cocker when she was a puppy. That was about 4 years ago.  They have always gotten along well and often play together.  However, in about the last 6 months we have been seeing some aggressive behavior from the cocker.  In the evening, when my wife and I are watching TV, suddenly the cocker will start "stalking" the hound, sniffing at her, and then they both start growling and soon they are fighting - viciously!  If we try to separate them when the "stalking" starts, they start fighting right away.  Both of them will snap at us when we separate them while they are fighting.  Sooner or later, one of the dogs, or one of us, is going to get badly bitten or hurt.  Strangely, this only seems to happen when my wife and I are both home and in the family room. If my wife goes upstairs, the hound will follow her and there is no problem, but we cannot allow the dogs' behavior to determine when my wife and I can spend time together! What can we do to stop this behavior before it gets to the point where they are fighting?

ANSWER: The first thing I'd do is get your cocker a total Wellness check PLUS a thyroid panel.
Any time there's a sudden, inexplicable behavioral change physical cause should be eliminated first.  The thyroid panel is important and your cocker is just at the age that can happen.

So this only happens at "tv time"?  Where are the dogs?  On the furniture? On you? Are they fine at other times?  

What's going on is that you're not "interrupting" the behavior but trying to stop it once the dogs are in full flight.  The time to correct is at the first sign of "stalking" and stop it then.  How to do that?  You can leave a short, light leash on her that you can pick up instantly and be in control.

Or you can fill an empty pop can with 15 pennies - duct tape the top shut and have it right beside you.  When she begins to stalk - pick up the can, raise in the air and bring down hard ONCE and say NO!  The idea is one thundering noise and the message is simple "You do THAT and THIS happens".  No further comment - no chat - no scolding.

But this is very curious for dogs who've always gotten along.  There seems to be some territorial/jealousy thing here that for some reason is triggered at this particular time and place.  Which is why I'd ask specifically "where" everyone is - who is getting attention from who? Are you on a sofa together?  Separate chairs? You say if your wife leaves, the hound follows and then it's all fine.  So I'm thinking she is your wife's dog and the cocker is more bonded to you?
Delores

---------- FOLLOW-UP ----------

QUESTION: I never thought about the issue being physical.  When the behavior starts, my wife and I are in different chairs, the cocker is usually in my lap (sometimes my wife's lap), and the hound is usually on the floor on her blanket.  Yes - they are usually fine at other times.  I actually tried the can w/ coin method last night and it seemed to work at first, but suddenly they started at it and the can didn't seem to make a difference - although there was some hollering and attempts to constrain them during that time.  Yes - we were thinking that it is some type of territorial/jealousy thing, but everything will be fine for an hour or two and then suddenly it starts.  You are correct in your assumption that the cocker is more bonded to me.  With the hound, she really seems to have no preference.  If my wife goes upstairs, that is the hound's signal that it is time to go to bed (both dogs sleep in our room on the floor) and the cocker stays with me.

Answer
Okay...you're in different chairs..cocker in your lap..hound on the floor.  

Then what happens?  Does the hound approach?  Does your cocker suddenly get off and start up?

I'm thinking it may be that she's ON you.  This could be what's called Lap Guarding and it only happens when one dog is actually on an owner's lap.  They do sleep in the same room BUT on the floor !

For the moment (until we can figure out just what this is about) I'd leash her.  This, of course, is unacceptable behaviour but if we can figure out the trigger, it'd be extremely helpful.
Delores