It depends, really. If you introduce the dogs properly and subside over every single time they interact with each other to ensure safety, then yes, they should learn to cohabitate comfortably. Whenever we introduce a new dog to our family, be it dog sitting or a permanent addition, I'm always vigilant for signs of uneasiness and tension. If I sense tension or the dogs are avoiding each other, I try to find what they're upset about. Remove it and gradually re-introduce it as times goes by and the dogs are familiarized.

If your dogs have not had the proper introduction yet, I would recommend you do so even if the new dog has been living with you for an extended period of time. Also, you must remember your resident dog has been the 'only child' for a while. It's very threatening to his stance in the pack when a new dog comes in and marks the areas in which he claimed his territory (your backyard, for example).

Have their scuffles ever led to bites and blood? When they start scuffling, is it usually over a valuable resource [toys, human attention, food, etc.]? You probably know already, but it's a good idea to remove all resources [mentioned above] if you're not there to supervise. My resident dog was a tad food/toy aggressive, so I made sure that my new dog was NEVER allowed access to my resident dog's toy. If she was given access, my resident dog would have gone into attack mode and the results would have been devastating.

Is your resident dog alpha? If so, then I would direct my attentions to the resident dog and make sure that my resident dog receives every resource first.

But to answer your question, yes, they should learn to live together peacefully. My two dogs were pure h-ll when I first brought my Greyhound into my home. Lucky was so territorial and I struggled for the first few months to suppress his urge to challenge my Greyhound. It got to the point where the Greyhound was SO scared of my resident dog that she wouldn't bear to be in the same room with him. They both became withdrawn and my resident dog was *very* depressed. The Greyhound was timid and painfully shy. It was then that I questioned myself, Should I relinquish the Greyhound? Gradually, but surely, the two dogs began to tolerate each other. Now, 9 months later, I sometimes catch the two sleeping with each other.