Introducing 2 dogs is a lot easier than introducing 2 cats, I''ll tell ya that much!
When we introduced my golden retriever into my boyfriend''s "pack", we decided to go about it differently than just throwing him in and seeing what happens. He''s very timid, and my boyfriend''s chihuahua/rat terrier mix is a little @$$hole. (So funny to see my 98lb golden run away from his 15lb dog).
I walked my dog, and BF walked Charlie at the same time. We did this on territory that neither dog had ever been to. We started far away from each other, and got closer and closer, until we were close enough to have a conversation, and the dogs were close enough to smell each other, but not touch. Stayed like that for about 15 minutes, and then took my dog to the house first. BF came in with Charlie and kept him on a leash but sat on the couch with him and talked to him, while my dog wandered around the house. We watched a tv show and after that we took both dogs outside to meet for the first time. Supervised, of course, and both on leashes. After about 10 minutes of satisfactory behavior, we let them off the leashes. There were a few squabbles initially, so we stayed with them, correcting bad behavior and praising the good. Well actually they weren''t squabbles, it was Charlie biting my huge dog on the nose repeatedly and my dog running for the hills.
Long story short, if your dogs (or one of them) has aggression issues, it''s best to do scent before touch, and ease them into it. Charlie can still be a pain, but if we hadn''t done it that way, I''m sure it would have been worse. But MOST dogs are very friendly and just throwing them in the backyard together is fine.
Something to remember though, your dog may be friendly with other dogs he sees, but when you bring another animal into your home and you split the attention and your dog realizes it''s a permanent thing, he may feel differently. Always monitor the initial situation for a few days. Fights can start and escalate fast over things you didn''t know were problems. They will work out the heirarchy (sp?) at first.
Don''t get me started on introducing cats! I spent 3 DAYS gradually introducing my kitten to the pack, and one of my cats STILL won''t speak to me, and it''s been 2 years! ARG!