Please don't get 2 dogs of the same age at the same time, I.e. not litter mates. Google litter mates syndrome. It's awful.
Otherwise, if you have 1 dog and are looking to add another, then it can be great, although my experience is the second dog doesn't get so much of a look in, and it's more tricky to train as you usually have them both out together. Ideally, you want to spend a lot of 1to1 time with the new dog.
If the first dog is getting older, it's nice to have a second one so you are not left dogless, which for me is very difficult.
Equally, is the first dog is very old, then it's not very fair to introduce a young puppy, as that might be too much for the old dog.