I would advise waiting until your younger dog is at least 2 years old, a a little more mature, but also by that time your oldie will be a lot older and not so easily able to cope with another youngster in the family. It is a bit of a dihelima as to what to do for the best.
We have a 11 year old jrt and a 3 year old border terrier, and a 9 month old border terrier, they are a very happy little family. You know your dogs best, if you think that your oldie would not cope, it is kinder to not get another.
My pup is play's lots with the border 3 years old, but now is inviting the jrt to play as well, he seems to enjoy joining in even at his age, and the youngsters have given him a new lease of life.
We had to think seriously if we could afford another dog as well in this presant financial time we are in where there is credit crunch everywhere.
good luck with what ever you decide