Thanks for the additional information. It is helpful.
Let me explain what I think has happened. Usually when you get a new puppy or younger dog, the older dog in the house is considered the boss or alpha dog. As alpha, that dog gets certain privileges such as eating first, getting attention first (they are allowed to shove the other dog out of the way), going out first and in first, and even deciding where they want to sleep, etc. Pups usually accept this naturally and there are no issues between dogs. Many older alpha dogs don't push a lot of the privileges but they still usually want to be respected.
Now what ends up happening is that a younger stronger dog matures and if the alpha is an older dog like yours, the younger starts feeling like they are a better choice to be the alpha dog. This is something that often happens around 18 months or so. Some older dogs will just accept a change in the boss and their new lower status and the lack of privileges they used to enjoy, but others don't accept it and will attack the younger dog to try and maintain their place in the pack. If they do just accept the switch, often the new alpha (younger dog) will feel the need to continue to show the older dog that he is no longer the boss and ends up reprimanding the older dog for not changing the way he acts. For instance, they might have frequently changed sleeping spaces often in the past and it was the older dog's choice so the younger dog did as they were supposed to and obeyed the older dog's wishes. If the older dog tried that when they are no longer the boss, then the younger dog would feel they needed to reprimand the older dog because it is now the younger dog's choice as to where he sleeps. I do see this type of in fighting among dogs in a household fairly often.
Now there are some things you can do to help the situation. One would be to get full exams done to rule out medical conditions. Younger dogs often attack sick animals and dogs with hypothyroidism often start attacking other dogs.
You will also need to change how you interact with the dogs. The alpha dog (in this case it is likely the younger dog now) should be given privileges such as I discussed. His food should be put down first, he should be given attention first and be let in and out of the house first. If the older dogs pushes past him, ignore the older dog and give the younger one the attention. Give treats to the younger dog first. I know if is tempting to give your old friend the attention but when you do, the new leader will take it out on the dog because they can not take it out on you. So keep this in mind when interacting with them.
The other thing is that they are going to need obedience training. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Once trained you can use commands to control their behavior and hopefully stop fights before they get bad especially if you learn body language and can give the command before it escalates into a full out fight. Read about body language here:
Additional training can also help. Both dogs should be leashed and if one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack.
As far as sleeping arrangements go, you will want to only allow the alpha up on the bed. That puts him higher than the older dog. The alpha dog usually sits and sleeps higher than the lower ranking members of the pack. If the older dog attempts to switch, don't allow them. If it makes you feel better, get a comfy bed for them on the floor.
You might also consider basket style muzzles for them until you have some training done. Basket muzzles allow a dog to breath properly and often drink and even eat a bit. It will prevent bites. Only use them if you can not have them leashed and watch them closely. It will allow interaction between them without allowing bites. See one here:
If you use these techniques along with training, you should be able to reduce the altercations considerably.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.