I was hoping to get some response back to my information request but don't want you going without an answer.
At 6 months of age, the yound dog will be maturing into an adult. As dogs mature into adults, they start challenging those in charge. In this case, the older dog would have been seen as the boss dog while the pup was maturing. Now you have an elderly dog who likely has some health issues and isn't the best choice for the top dog position, so the younger healthier female is attempting to take over the top dog position.
Unfortunately, this is how dogs react in a multi dog household unless there has been a lot of obedience training and supervision. In most cases it is best to allow the dogs to work this out on their own, however, the older dog may not feel like giving up his position in the household. Top dogs usually get to eat first, get affection first and treats first. Once they are no longer the boss, they often will continue to act like they are which causes the new top dog to attack them and reprimand them with a growl or physical attack.
So if you do not know of a medical issue with the older dog, I'd get him in to be checked over as it is likely he has a medical issue brewing. You should immediately start obedience training with the younger dog. It needs to be structured training on a daily basis. 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.
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.
Your girl needs a lot of training to get her under control. 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.