I have 2 dogs (an almost 10 year old golden retriever/lab mix we got as a puppy and a 2 year old shepherd/collie/doberman/who-knows-what-else mix we got a little over a year ago when she was 11 months old). I also work at a shelter and I have people in your situation talk to me on a daily basis.
Without knowing any details of your dog (is it male or female? fixed? around other dogs ever? ) an 11 year old dog probably has no interest in a puppy. A dog that age does not want to deal with crazy, playful puppy. They are more interested in hanging out and playing only when they want to play. Whenever someone comes into the shelter asking me about this, I try to recommend slightly older dogs that are on the calm side. I don't mean I tell them to get a 9 or 10 year old dog, but I do try to recommend dogs out of the puppy stage and explain why. If they still go for the puppy, that's fine, but I then explain to them that the puppy and the older dog will not be playmates. You may say that your dog is energetic and playful (which he may be) but you don't realize the difference between a puppy and older dog until you see a puppy. My older dog was very active, very playful (or so I thought). I got my younger dog home and realized just how much the older dog had mellowed over the years. Over a year later, we have finally gotten to to the point where they will tolerate eachother, but will not be best friends (EVER!!!) and will not be playmates. If you want to get another dog, go for it. You just have to go in and be ready to accept that the dogs may just be able to cohabitate and will not be best friends. You should also take into consideration your older dog. I am all about shelters. There are millions of great dogs in shelters looking for a good home and the older dogs (I'm talking dogs over like 3 months) wait so much longer for homes.
And I just saw your second post, be very careful with a daschund! They typically are not good kid dogs and your bigger dog could be very dangerous with one. They have very delicate backs (because they are so long) and can be easily hurt.