Ladymom posted this link in another thread yesterday, and it had some very interesting advice on choosing the sex of the second dog. From reading this it sounds like what you are going through is typical. You want them to be best of friends, but this doesn't always work with two dogs of the same sex. The article also suggested that alone time with each pet is important, just like real kids. For more information:
Two Puppies or One? - VeterinaryPartner.com - a VIN company!