Is there any chance you could introduce them to each other on neutral ground. That way neither is feeling territorial or threatened and you can at least see if they ignore each other, play or show dislike. if all went well then try it a couple of times before committing to taking the younger dog.
It really is an individual choice, personally me and my OH have always prefered keeping two as we feel they have a companion when we need to pop out or just to share checking out smells on walks, or having a rough and tumble.
We originally (20 years ago) had two sisters who adored each other, next, after losing one we had a pup who ended up being mothered by the older dog. Next we had that grown pup (aged 8) and a year old rescue, (treated very badly) they were okay with each other but did not feel the need to cosy up together. Now we have that rescue (aged 2) and our new pup again. The older one will always be a bit reserved I think but she definately likes the new pup and they are closer already as they play constantly together. Theirs will be a close bond, although the pup who is now 5 months is already the boss! I do think the fact that we had a puppy helped as the older one with a history did not feel threatened by a younger dog.
So as you can see it is a gamble. But it definately helps if they initially meet on neutral ground. If you did decide to take on another dog then you want to give them the best start together to lead on to a good relationship between them.
Good luck whatever you decide.