I don't even know where to start...
In the nicest possible way, please read this and this before getting a second dog. Your dog doesn't think you are pack leader, dogs do not think like that, she thinks you are a resource. She will not worry that you don't love her if you bring in another dog, she really doesn't understand love in the way that we do, she understands that you provide food, fun and comfort (i.e you are a resource) she may start guarding her resources from the second dog if things are not handled properly, depending on her nature and previous training. Please do intervene if they get into a scuffle, in fact, intervene before they get into a scuffle. These things can and do escalate if they are not addressed properly, sometimes to the point where it can never be rectified.
The best way to go about it would be to approach a rescue, who will asses both your current dog and your family and advise on which dogs they have who would suit your current set up and who should provide back up and help with any teething problems you have.
I found the biggest change when I went from one to two dogs was the increase in noise. Play fights are not quiet. I'm sure at times my neighbours must think I run a dog fighting ring in my dining room.
If you read up on canine behavior you'll be able to spot any problems (resource guarding is the most common in multi dog households) before they start and work on solving them.