When I brought home my second dog I encountered similar problems. The two just didn't seem to like each other at all. I have always crate trained my dogs, so I put the new dog's crate right next to my other dog's crate. For the first couple of days, every time I had them out in the house or the yard they growled and snarled at each other. Instead of trying to force them together, I started rotating them in and out of the crates. I would take one out for an hour, and then take the other one out for an hour, and so on. At night, they both slept in their crates and any time I wasn't home they stayed in the crates. After a couple of days doing crate rotation, they both seemed to just get used to each other. I started letting them out at the same time (always supervised of course) and they were fine. No more growling and snarling. Now the two are best friends! Having the crates next to each other gave them a chance to get used to seeing and smelling each other without any pressure to interact (plus I didn't have to worry about a fight breaking out!). I can't say for sure that this will work for you, but it really worked for my dogs.