Even though this is an old thread, I thought I'd add my 2 cents
We had one for a couple years and I really wanted a second. We added a female to the mix, and couldn't be happier. I sometimes think about how much easier it was to travel with one, but we spend more time at home than traveling, so it's great to have two... They play together, love each other and keep each other company. I have one independent pup and one needy one. It has worked out. They are a good balance and I cannot imagine having only one.
Just remember, two pups means double everything... Double the vet bills, food, treats etc.