If you wait until your first dog is at least five to six months old before you get a second one, your first dog will be well bonded to you.
If you get two puppies at the same time, especially if they are from the same litter, they are likely to bond to each other and pay more attention to each other than to you. Training them will be a lot harder! That said, I know some people that have gotten two puppies from the same litter and successfully raised them to be well behaved dogs.
Information from How to Raise a Puppy You Can Live With.