There are pros and cons to both...getting a second puppy will allow them to grow up together, and they may be better bonded. However, you may find they're difficult to train, since it will be hard to tell who had the accident in the house
, and it's harder to get TWO squirming dogs to "sit" than just one. My two are about 6 month apart in age, and that is working well for us-We brought Milli home (she was 12 weeks old) when Emma was 11 months. They bonded right away, but we didn't have the difficulty with potty training issues with Milli since Emma was already potty trained and knew lots of tricks before Milli came along.
I also must caution you about purchasing a dog from a pet store. Pet stores are known for selling puppy-mill dogs. It's a vicious cycle between the so called "breeders", the brokers, the pet stores, and the buyers. I know you may see them and think they're so cute and that you've just got to have one,
but realize that by purchasing a dog from a pet store you would be supporting puppy mills. Also, you may find that you can get a quality pure-bred puppy from a reputable breeder often for the same price or less than the price at the pet store.
I encourage you to do some research on the topic, as well as on chihuahuas as a breed, and look for reputable breeders in your area. Then you can make a decision on where and when to get a chi puppy.