Get a dog because YOU want another dog and not for the other dog.
We are one week into having a new puppy, a cocker spaniel. Our springer spaniel is 15 months and very well trained (he has his KC gold, which was the standard we wanted to get him to before contemplating another)
It's bloody hard work but I wouldn't swap it already
I have done nothing this week but devote time to the two dogs! Made sure Ddog1 has bags and bags of individual attention, training etc.
Ddog1 ignored the puppy for 48 hours (he's also quite reserved), then decided he was pretty good fun, now he's very good fun and they love each other! I haven't seen a slip in behaviour so far. The puppy is definitely bonded with us first, but he's a much more friendly soul. He doesn't pester ddog1 though, which helps. They sleep together during the day, then we crate the puppy at night. Although, DH left the crate door open last night accidently and they slept together 
I train them both together too, with the clicker, just reinforcing commands for the first dog or giving a different cue.
We seem to have been lucky so far, but you just have to devote all your time to making it work.