Yaaah we had this problem when we got Chester. Ruby was about six months old and we'd had her for three months, and when we saw Chester we fell head over heels. Many people here gave me great advice, and I would say the best of all is to wait until your pup is a bit older. Ruby was already toilet trained and had her basic commands down before we got Chester. It was (and is) easy to get her attention when the two of them are together, because she was here first for a few months. This can be much harder when you get two pups together because they can and will gang up on you, usually because they're so absorbed with each other they don't notice you're there!
Also, if you let your current pup get quite a bit older before you get another, there won't be as many problems with separation anxiety should you need to separate them for any reason. It is always best to train them to learn to be apart anyway, but this is MUCH easier if the first one already knows how to be by itself.
Other than that, getting a second pup has been the best thing we did. Chester is beautiful, he and Ruby completely adore each other, and they look after each other and keep each other company. It hasn't been that much more work, and it's just been the best thing for our little Rubes. Dogs need company, IMO, and two dogs are much better than one!
Oh - and a different sex to the one you have now is definitely the best idea, when the time comes for another. But again, it would be much better to wait.