Cindy's Answer

It's impossible to know for sure but when dogs have a long history of being enemies those feelings may always be there. Muzzles will hopefully keep them from being able to bite each other but it doesn't do anything to change their attitudes towards each other. In some cases, it can make dogs dislike each other more as they become frustrated when wearing the muzzles.

If they had only been in one fight and you had tried the muzzles and lots of training with them together at the start of their relationship, it might have had a better chance of success. There are just some dogs that won't ever be able to live harmoniously together, especially after repeated fights.

I think the best you could hope for based on the short description of the history, would be for them to be neutral to each other.

I could be wrong, of course. This is just an educated guess based on lots of experience helping folks with dogs and with having a couple of dogs of my own over the years that simply just didn't mesh with my others. Keeping them separated is a real pain, I know from personal experience. You're always at risk of a management failure too, someone leaves a door open and you have another altercation. It's stressful!