i heard this rule of thumb many times and just don't get it as long as your 1 year old is properly trained and socialized/loves other dogs. just wanted to share my experience and hear about other experiences on the subject.
i totally disagree with the 3 year rule. as long as your first dog is highly trained and loves to be around other dogs.
First off I'm going to say I've got dogs that close together so I'm going to agree with you that it can be done.... However, the down side that I've had to deal with, more than once, is on the other end. You loose them close together. Its hard losing one. Having them both pass within a short time is absolutely killer.