It completely depends on the personalities of your dogs. Wosie was almost two when we got Dreama, who was 4 at the time. Wosie adores her sister! Dreama, who was raised to be an "only child" would be happier if she still was, I think. She doesn't play with Wo a whole lot, and keeps to herself more than hang with Wosie. But I know they love each other, they just love their peeps more than they love each other.
It's my hubby and I who can't just have one dog. We each are happier with our "own", even though we love them both. Wosie has always been a momma's girl, and Dre is Daddy's. Therefore, there is no question in our house. There must be two!
Get the second dog, and see how it goes.