I'd wait a little longer, personally, for a few reasons. First, I don't want two senior dogs at the same time- so much money for health problems, and so much heartbreak all at once.
The general rule of thumb is when the first dog is mature and training is down pat- no issues needing to be resolved, etc. Your dog is still pretty much a puppy, so you'll have two really high energy, high maintenance pups on your hands.
We just added a second dog, my male is four and I'm just now reaching the point where there's nothing I have to worry about training/behavior-wise with him.
Delta zur Bindenburg- 11/15/13
Gentle Tripp- 6 yr old OTTB mare