Increase these durations of absences very slowly and go back to shorter departures should the dog show even low levels of anxiety. Once you can safely leave the dog for a minute or two, increase the intervals of separation randomly so that the dog cannot learn to anticipate when you will return. Remember the goal is to gradually accustom the dog to being alone without becoming anxious. If your dog suffers from separation anxiety, I encourage you to have your veterinarian recommend a behavior modification specialist. An appointment with a specialist could ultimately save the dog's life and your sanity.
Separation anxiety can be a very complicated condition.
One of my Greyhounds, Midnite, suffers from extreme separation anxiety. Change in food to one with lower protein content. An antidepressant - which we tried when all other courses of action failed. For a dog of this type, the presence of other dogs may mean nothing. Whatever you do, don't give up.