We went from one 9-month-old to a 9-month-old and an 8-wk-old...then shortly thereafter added a 16-week old. A few years later we went from 3 to 6 between the ages of 18 months and 3 years. A few years after that we went from 6 to 8 by adding two dogs a few years younger. There were a few brief rough spots going from 3 to 6 members, but all-in-all the Pack adjusted quite quickly to each change.
As long as the dogs get along, they'll likely enjoy each other's companionship. Is there a way to have them meet in a neutral place first and then see how they interact with each other at your home on a play-date before you commit to taking the newbie?
Now, what would you be in for? Lots and lots of rough-and-tumble play, probably.
At 10 and 11 months, our two pups were definitely a handful. We were fortunate to have a small but fully fenced and puppy-proofed backyard. Even so, when we moved, we needed to totally re-landscape the back because the grass, garden and small trees and bushes had all fallen prey to puppy shenanigans and horseplay. Granted our dogs are a fairly high-energy (at that age, anyway) breed and larger than yours, but I would not want to take on two young dogs of that age without a safe fenced area to allow them to play in.
You'll also need to work out a schedule that allows for training them separately as well as training them together. They'll bond to each other pretty strongly at that age and one-on-one training will strengthen your
bond with them so they look to you
, not always to each other.