It really does help if your first dog is mentally mature enough and trained enough. That might be at 1 year or 2... depends on the dog and you. In the mean time what you can do is work on his doggy socializing skills. Have him hang out and play and interact on a regular basis with other dogs. The key thing is that you should know these dogs to reduce the risk of introducing your dog to an aggressive dog.
Also, don't get the second dog thinking the pup will keep Toro company while you're away. The dogs should not be left alone unsupervised, at least for the first year, two, or so. Again, it will depend on the dogs. For starters, you could be sacrificing house training progress by allowing both dogs free run of the house too early. Bigger issue would be the fact that at the age of about 1 or 2 years old, the dogs are comming into maturity and this is the prime time for possible dominance conflicts to arise. You don't want them to have the opportunity to get into a fight while you're not home and they're running around loose in the house.