I think that Sheila has said most of what I would have.
I kept my dogs seperated until my male was 2 years old. She just would not tollerate his crap at all. Other than for walks together after he was about 6 mo old. But we walked & there was not silliness tollerated. No posturing, no hard eye contact no physical contact was allowed. We walked, period. I would physically control the pup & just not allow him to be bugging her. She would have just taken him out if he pushed her too far, so it had to not be allowed to start.
By keeping them seperated,(she was already 3) he became bonded to me, not her. I work them together & seperately. There are some pack mentalities that go on, but I am the center of their world.
Groundwork is very important & many people overlook the value of it. Many people find it easier to let the dogs pack up & work things out. It is not fair to any of the dogs to do this. But it is the general public's way to do things. Ergo: dog parks etc. Sooner of later, some dog pays the price.
I would work your dogs seperately until you get to the point that your dogs look at you when ever it is time to train, go for a walk etc & not around for the other dog. Eventually, I would even use one dog in a crate within sight while training the other dog as a distraction. It may take months to get that kind of individual bond with each dog since it has been 1 1/2 yrs. But it could be sooner, depends on the dogs.
I would not take them for walks together, though. I would not rush it. I know it is double the work, but it usually is with multiple dogs. Possibly you could eventually get someone to walk with you & take their dog & you take one of yours & look for the dog to pay attention to you only, not the other dog & see how that goes. At some point, maybe you & hubby can take a short walk together, even if it is just up & down the driveway, each with one of the dogs & see how that goes. Or have someone else walk one of the dogs & you with one. If the dog started to focus on the other dog...ask for a 'watch me' command, (teach this)to get their focus back on you.
You will have to just take the time & work at it & see what works & how it goes.
I honestly never had this problem, because it didn't allow it from the begining....but this is how I would go about it if I had to take it on or help a friend work thru the problem.
Hopefully it helps. There may be others here that have worked thur this with their dogs & can help you more.
ETA.....it is possible that you may never get these dogs to 'get along' together without the younger one bulling the older one or at least trying to do so if they are free together. But I would sure put in the effort & try to see if it can be worked out. If you can eventually walk them together & would stick with this for a few months before ever trying to have them out together in the house & when I did do that, they would both be on lines with you on one & hubby holding the other line. I would travel do this road for a long time before letting go of the lines one at a time for a while & if all goes well eventually the other loose too. But you may always have to be vigilent watching these dogs. One goof & they will be back to square one again.
Good luck.
MY DOGS...MY RULES